What other webassist extensions do you have installed? The function referenced in the error isn't used by the latest version of the extensions, so maybe old files were left behind somehow.
You may need to:
1) close Dreamweaver
2) Rename your User's configuration folder to something else like: Configuration_Bak
3) Reinstall the webassist extensions
4) Re-open Dreamweaver and let the secondary installer complete
5) Restart Dreamweaver a final time
If that doesn't work, post back with a US phone number or Skype contact information and we will set up screen sharing to debug this with you directly.