First: You need to make a unmodified Minecraft.jar do not install Modloader.
Second: You need to run decompile.bat and once that is done execute enable_modloader.bat once.
Third: Create you mod/mob as you would normally.
Forth: Reobfuscate the classes, but do not include Modloader .class files
fifth: Now take the release of the Modloader .class's from Risugami and use them with your mod.
Sixth: Run test_modloader.bat not test_game.bat
Note:This is a basic Tutorial, and is for the users who are starting with Mcp or modding.
Note:You can also look at README-ML in the Mcp folder for easier instructions.
^ It too me a while to figure this out. I'll assume you know how to use MCP programs and where to place what. You will need your own copy of ModLoader (preferably v4) and spawnlist. Moving on:
1. From your mpc folder, in the directory *\jars\bin\minecraft.jar, delete the META_INF folder.
2. Place the following files from modloader.zip, as well as spawnlist.class, the files for the mod, and any other files the mod needs, into minecraft.jar: BaseMod.class, ModTexture.class, ModLoader.class
3. Decompile the files.
4. From yor MPC folder directory *\sources\minecraft\net\minecraft\src, delete the following files: BaseMod.java, ModTexture.java, ModLoader.java
5. Run enable_modloader.bat
From there you should be able to hack away. A select few mods requiring modloader have given me trouble with this manner of fiddling, generally ones that alter existing classes rather than just adding them. But this otherwise works fine.