SU2013 has required that the plugin's entire user interface be rewritten using different technologies. Thankfully, that work was already started many months ago. Please see here
for information (generally, we discuss SketchUp-specific topics in that part of the forum) on a recent update that introduces the new user interface. That is just for SU8, but will also work in SU2013, if you are using the Standalone Maxwell for SketchUp plugin.