The message from Exception is:
"java.lang.UnsupportedClassVersionError: "nameProject or plugin": Unsupported major.minor version 51.0
This plugin requires Java 1.7 or later"
This happened when you compile the classes with an IDE such as Eclipse or Netbeans and that you did not change the JDK to 1.6 but left it at 1.7 (AKA Java7).
Two possible solutions: switch the IDE to a lower JDK, or run ImageJ with a newer JRE.
In my case, when I had this exception, I did the first option.
I use NetBeans and put a lower JDK, for this: right click about the project and click in properties.
In Source /Binary Format change of JDK.
Now, run the plugin other time and... ready!