Hi, maybe the problem is that you have a java 1.6.0_20 version working with imageJ, you can see it if you go to your folder installation of imageJ, look for a java folder, rename or delete de folder and restart ImageJ and prove the plugin.
i prefer to delete the folder because this way ImageJ work whit java version that you have installed in the computer.
http://imagej.nih.gov/ij/docs/install/windows.htmlLook for - Upgrading to a Newer Version of Java
Greeting