I uninstalled all WA extensions, uninstalled CS5, used CS5 Cleaner and cleaned the registry.
I then performed a new install of CS5, performed all CS5 updates, and installed Site Sculptor. Still no preview in the extension window. Browser preview is fine (available regardless of the condition of the "Disable Preview" checkbox in WA preferences), but extension preview is not available regardless of the state of the Disable Preview checkbox.
The message in the preview box says to click to preview in browser; it does not have the "generating preview" message.