Apple's new Mac App Store is clearly a work in progress, and as a first release the current version, sometimes has a few quirks with handling installed applications, among other issues like handling authentication and account sign-in. A few people have recently contacted us regarding the ability to sign in to the App Store, with problems from the store refusing passwords, to the login window not accepting any input.
If you are having troubles logging in to the Mac App Store, first try testing your Apple ID. This can be done a variety of ways, including logging in to the iTunes Store, or by checking and managing your account at Apple's account management Web page.
With your account verified, try the following options in order to see if they help fix the issue:
Clear the App Store cache
Go to the /username/Library/Caches/com.apple.appstore/ folder and remove the file called "Cache.db." Then relaunch the App Store and try again. You can also try removing the entire "com.apple.appstore" folder from the caches directory and retry the store.
Clear Store preferences
As with other applications, corruption in the store's preferences file may result in odd behavior and may affect functions like logging in. Try going to the /username/Library/Preferences/ folder and remove the file called "com.apple.storeagent.plist." Then relaunch the App Store and try again.
If the first two options do not work, try opening Disk Utility and running a full permissions fix on the boot drive. This should ensure all system files are properly accessible by the system and hopefully correct any error with file access that could be contributing to the problem.
Run combo updater
Lastly, if the App Store application or a required system component is corrupted, you should be able to overcome the problem by reinstalling it. Go to the Apple Support Downloads page and get the latest combo updater (currently OS X 10.6.6). Restart the system into Safe Mode by holding the Shift key at boot-up, and then run the downloaded installer when in Safe Mode.