Hello, I have and have been using the mm8leveleditor to modify the games 6-8, don't know if it works on 9, I only played 9 brefly and didn't like it, but I love the other three and although I have figured out many things to mod there are still many things I'd like to do so I thought if we all pitch in what we know maybe we can all get the games to do what we like. So here's my mini guide, anyone else feel free to add to it or download the editor and start messing with it yourself, maybe you'll figure out things other's can't:
1. What you need to get started - go to http://sourceforge.net/projects/mm8leveleditor to download the editor, follow the directions to install.

2. Here's a list of things I currently can do and things I can't do and I will explain how to do each. Can do:
Edit strength of monster's, things like HP, AC, Spells, etc.
I can edit weapons, their strenth, their look, even what kind of weapon they are, ie: I can turn a sword into a spear, a staff into a ring, etc.
I can edit sounds, pretty much any sounds, want to change the voice of character, or the noises a monster or weapon makes, I can show you how.
I can modify what monster's appear on a map or dungeon, for example let's say in MM8 in "Garrot Gorge" you want Naga's replaced with more dragons, I can show you how. Portrait's, I can change the portraits of any character or NPC.
Can't Do:
Although I said I can mod weapon's, there are some things I can't yet figure out, for example I can't change the power's a relic/artifact has, I can still change a relic/artifact's look or even change them into a different kind of weapon, but the power's stay the same. I also can't change the abilities of a weapon type, ie: I can't say at Expert level axe instead of getting a speed increase you get an AC increase. Lastly modding armor - you can mod armor's stat's such as it's AC, but even if you change the item picture, once you equip the armor on the player it will look like the original armor that it was, the only exception to this is the shield.
Another thing I can't do is move dungeons around, although I can change the creatures found in dungeon, the dungeon itself will stay the same and will appear on the same outdoor map. One of the things I wanted to do in MM6 was move the "Werewolf Lair" to Darkmoor and swith it with Snergle's cavern's, the only thing that happened was that creature just switched places, but the dungeon's remained the same.
I can't mod the classes, I can change the text regarding the classes, ie let's say you want to call knight's Barbarian's and make Paladin's knight's, you can change the wording however that is all. One thing I want to do for example is make Archer's pure fighter's and take away their spell points and add them to hit points, however, I can't figure out how.
Music: Although I can edit sounds, I haven't figured out where the music file is, or a way to assign certain music to certain places.
Finally, I can't create character's like you see in MM8. This is something I would like to learn how to do.

Now, on to the tutorial.

Warning: Backup any files before you edit them so if you make a mistake you can easily just restore your work.

1. How to edit items - You will need to edit the "Items.txt" file, to do this lauch the MM8level editor, go to whatever folder you installed your game/games in and find the "Data" folder. If you are modding MM6 open the "Icons.lod" file to find the "Items" file, MM7 it is in the "Event.lod" file, for MM8 it is in the "EnglishT.lod" file. Select "Items.txt" then go the extraction tab and extract the file, you can specify where you want to extract the file. Once you've done this go the folder where you extracted it and open the file, from here thing's should start to look pretty obvious as to what you want to do. If you want to change one sword to look like another, simply change there item number, IMPORTANT - you must also change the numbers that dictate the placement of the weapon when you charcter holds it, if you don't the character will hold the weapon, but it will be all out of place. An easy way to find these number's is to scroll to where the item description begins, there will be 4 number's preceeding this, these are the number's you have to write down.

How to change monster's abilities, much like changing weapons except you have to go to the folder that says "Monsters.txt", from there just simply do whatever you want basically, add remove a monster's spells, increase/decrease their armor class, it's all there, you'll get the hang of it.

How to add/remove monster's from a map dungeon. Find the "Mapstats.txt" folder, like the other's, extract it, open it and it is pretty self explanatory. Just delete whatever monster's you want from a particular map/dungeon, and replace them with whatever you want, just make sure you use the correct wording, just because you see a monster listed as a tree while playing the game doesn't mean that's the name of it in "Mapstats"

How to edit sounds, now you have to get out of the data folder and to to (suprise) the audio file. Open it up, and there will be all the sounds, most of them are labeled fairly similar to whatever they are in the game so extract whatever ones you want to change and them rename them to the name you want them to replace. IMPORTANT - be sure you rename the sound exactly as the name of the sound you want to replace it with, you must you exact case as well, so if the name of the sound you want to chang is Hf402 you must name the replacement file with a Capital H a small f and the number 402.

How to edit portrait's, you should be getting the hang of things by now so for this you go to the icons.lod in the data folder, extract whatever portraits you wish to change and rename them AGAIN, rename them exactly, case sensitive and all.