Well, for one:
Site Assist is really not intended for modifying sites.
it is intended as a site creation tool.
once the site is created, you should use Dreamweaver as the tool for editing the site.
Site Assist itself has been discontinued and replaced by Site Sculptor which is now available as part of the Design Extender extension:
Design Extender will work in CS6 and Mac OSX Lion, but is not for modifying sites created by Site Assist. like i said site edits should be done using Dreamweaver