If you are having problems accessing Snow Leopard’s Mac App Store, where it hangs with a spinning beach ball, make sure you haven’t disabled Spotlight.
Spotlight can be disabled using a variety of shareware tools like Spotless and TinkerTook, or via the command line. Spotlight can also be disabled by adding the entire boot volume to the Privacy list in the Spotlight System Preference pane. The Mac App Store uses Spotlight to search the Applications folder to determine what is installed, and it’ll hang when attempting to do so if you’ve disabled it.
Another reported cause for Mac App Store hangs is having Certificate Revocation List enabled in Keychain Access (Keychain Access->Preferences->Certificates). This is disabled by default.