Off the top I can say that Design Extender includes the PowerCMS Builder.
extensions.php
Then see this thread:
showthread.php?t=26483&highlight=WA_localURLToFilePath
(You installed a stand alone extension after it was included in the grouped extension install. Please install in this order - eCart (if you have it) then the Data Bridge and last the Design Extender)
I had the same problem and this fixed it.