I followed these exact steps:
"1. setup MCP like normal and make it READY for decompiling
2. install Forge into the minecraft.jar inside your jars-folder
3. delete the 3 "XX-Integration"-Folders in your IC²-jar
4. install IC2 into the minecraft.jar
5. delete META-INF inside your minecraft.jar NOW
6. put the content of the lib-Folder in your .minecraft into the lib-Folder of MCP
7. run decompile.bat
8. now you have the source, but it contains a few minor Errors."