problem loading Mumble-Link Minecraft (Class not found exeption) [closed]
Solution 1:
Unsupported major.minor version 51.0 indicates that the classes were not compiled for the Java version you are using (in this case 1.6).
The jar you have obtained is only compatible with Java 7 or above.
You will not need this mod on a dedicated server. It is client-side only.
Also note that there are other preferred ways to ask for support. The Minecraft forum and github are monitored by me somewhat regularly.