Have the same problem.

My plugin:

public class TestPlugin extends com.eteks.sweethome3d.plugin.Plugin {
@Override
public PluginAction[] getActions() {
return new PluginAction[]{new TestAction()};
}
}

Application output:

Invalid plug-in jar:file:/home/msk/.eteks/sweethome3d/plugins/sh3d_mirror_plugin.jar!/ApplicationPlugin.properties:
msk.test.TestPlugin not a subclass of com.eteks.sweethome3d.plugin.Plugin