So I am getting that same error
FINER: THROW
java.lang.UnsupportedClassVersionError: net/minecraft/server/mod_IC2AdvancedMachines : Unsupported major.minor version 51.0
The only way to fix this is if the server is running 64bit java version 7? I am trying to use this on a host, verygames.us and they allow 3 versions.. minecraft vanilla, craftbukkit, and openmod. I like to run things on openmod cause the craftbukkit version they have the craftbukkit.jar hidden or something, I tried renaming Tekkit.jar to craftbukkit and gave errors.
So, I just deleted the openMOD install, uploaded these files, started the server with Tekkit.jar and it loads everything fine except the IC2AdvancedMachines and I can not connect.
Is anyone else using verygames as their host and got this to work?