Mac App Store works like trying to open iTunes via browser
Go to your home folder. (your user name under places on the left hand side)
click on Library
click on Preferences
delete "ByHost" which is a folder. 
delete "com.apple.appstore.appstore.plist" which is a file.
delete "com.apple.storeagent.plist" which is also a file.
restart
empty trash
open Mac App Store and it should work. 


If you're worried about deleting files then just make a copy somewhere else before you delete them.  

From discussions.apple.com