first of all - thanks for the awesome plugin! Good job.
I noticed that when using a theme framework (in my case Thematic), the xml file only works if it's saved in the PARENT Theme's folder. I looked up your code and saw that you're using TEMPLATEPATH constant to look for the xml file. This doesn't work with child themes, since the TEMPLATEPATH is actually the PARENT theme. My guess is that you need to look up the actual "current" theme, whether it's a regular theme or a child theme, so you can get the correct URL for it.
Please let me know if there is a work around it, or you need to update your code in order to get this to work.
Thanks again for the plugin!