I think the OP might have been confused by the thread title of [MC 1.8.1] IC² v1.337 SSP/SMP
IC2 1.337b was released December 4, 2011, and it's for Minecraft Beta 1.8.1 that released September 19, 2011 (also it's not IC2 "experimental"). If you're trying to install it for the full Minecraft 1.8 that released September 2, 2014, then I'm not surprised that it won't work.
You should be looking at this thread instead: [LATEST RELEASE] [MC 1.6][MC 1.7] IC2 Experimental
Ahh, that explains it! I ended up starting a 1.7.10 server instead since it wasn't working. Now it all makes sense. Thanks!