DW CS4 and EM 2.1 incompatible with Snow Leopard
According to the list posted by Apple, Adobe Creative Suite is still written in Carbon and has not been upgraded to Cocoa. Apple re-wrote the kernel in Snow Leopard going from 32 Bit addressing to 64 bit addressing. Rosetta, the emulator that runs programs written in Carbon, is installed only when needed and Adobe reports that DW CS4 and Extension Manager 2.1 do not function correctly.
I switched DW to 32 bit (in the Finder, choose the app and Select Get Info from the File menu -- there is a check box that allows you to switch from 64 bit to 32 bit) That will speed up DW but not fix the problem of EM thinking that Web Assist extensions are not complatible and refuse to install them.
This is not a Web Assist problem but rather an Adobe problem. No response from Adobe when Creative Suite CS4 will be updated -- they have to re-write each program in Cocoa for full compatibility.