Board Thread:Tutorials/@comment-80.1.145.235-20130421110730/@comment-80.112.144.255-20130806163149

Unsupported major.minor version 51.0 means that the netherores mod was compiled in a different java version (jdk7) then you are currently trying to run the mod with.

java is not backwards compatible it seems... therefor update your java and it will most likely work