First off, I'll start by saying that this will most likely not work on all mods, seeing as it changes one thing to make it compatable with the newest version of WoW.
The method will usually work on mods that used to previously work in another version of WoW, but has either been discontinued by the creator, and the effect of the mod (meaning what it does, how it does it, who it interacts with) hasn't been greatly effected. I.E a mod that remembers an opponents name and stores it as KoS <--- example.
Anyways, first, you have to go to your interface/Addons screen, and go into the unzipped version of the file you want to change.
Luckily you don't need any other tools to do this.
Now, in the folder where your specific mod is created, there will be a bunch of file names, usually XML's, LUA's. But the file type you want is TOC (time of creation, I think is what it stands for). Double click it, if you can't open use, use another program and use "WordPad" or "Notepad," generally most text type programs work.
Now, when the file opens, your first couple lines should usually have something like this:
## Interface: 12001
## Title: Asmo Honor Role
##
All you need to do is change ## Interface: 12001 to ## Interface: 20003
Basically all this is doing is changing the version of the mod (when it was created) to WoW version 2.0003 and above. Now, when you open up WoW instead of saying incompatable, it will work like normal, unless the mod itself no longer works any more due to programming changes in the game itself.