I am getting the same error. Does it matter the order of installing the extensions? I installed data bridge first then design extender.
Right now all the extensions that I have from webassist are
paypal ecommerce toolkit 4.5.4
jeffrey zeldman's web standards advisor 1.0.1
Design extender 1.1.1
data bridge 1.1.3
I had a second instance of paypal ecommerce installed when I reinstalled the design extender and data bridge.
Webassist.com Paypal eCommerce Toolkit 1.0.0
removed it and restarted dreamweaver. No difference.
I also have one non Webassist extension installed.
Any idea what I may be doing wrong?