The GoJara plugin uses a library named json.jar. This library was compiled in a fashion incompatible with Java 5. This causes compilation to fail (Openfire requires Java 5, not 6):
The offending jar should be replaced with a Java 5 compatible equivalent (with a bit of luck, it needs recompilation only).
The json.jar library that is included with the plugin was replaced with a variant that was compiled using Java 5
@Override annotations have been removed from methods that implement an interface (again, not valid in Java 5).