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.
I've been using forge quite a long time; Checked version thrice before posting error, checked it to be working, downloaded IC2 as said, error is not caused by any other mod [note : there IS no error, IC is just not visible or usabe]. Forge does not recognize IC2, only suspicious thing is that it 'cannot access IC2 files' as if file was corrupted, but redownloading IC2 does nothing to fix it.