What other extensions do you have installed? What version of Dreamweaver?
This error most likely means that your menus.xml file is corrupt. The easiest solution might be to clear out your configuration file entirely and reinstall all of your extensions. If you have too many extensions for this to be reasonable, then I'd need a US phone number or Skype contact information and I could try looking through your menus.xml file in a screen sharing session to see if I can spot the problem and correct it.