Jason,
Gave that a go already and the installer triggered but I still get the error.
I had an earlier version of Design Extender, so I updated to 1.1.9 and thought that would fix the problem - no luck.
I cleared the cache file and reset the DW workspace - no luck.
The other WA extensions I have installed, if relevant, are:
DataAssist 2.1.0
Universal Email 4.0.1
SecurityAssist 2.0.5
Cheers,
Doug