I have the same problem using the debugger for volume example project:
Invalid plug-in jar:file:/C:/Users/user/Application%20Data/eTeks/Sweet%20Home%203D/plugins/VolumePlugin.sh3p!/com%2Feteks%2Ftest%2FApplicationPlugin.properties:
com.eteks.test.VolumePlugin not a subclass of com.eteks.sweethome3d ...