Ok, since I don't know what Hadaniel's exact error is, I'm going to walk you through it
1. Only install Forge (assume you know how to do that): http://files.minecraftforge.net
2. Start Minecraft, and verify that Forge works.
2. Download IC2 1.112.xxx, and place it in the mods folder unchanged (delete all other mods first, to verify that other mods aren't causing the problems).
4. Start Minecraft. If IC2 doesn't display, look at ForgeModLoader-Client-0.log inside .minecraft for any suspicious stuff. See if Forge even recognizes IC2.
5. If you suspect an error, post it in Support.