Hi,
You can try the 10.6.7 combo update
http://support.apple.com/kb/DL1361
but if that does not reinstall the Mac App Store for you there's another way.
Keep the .dmg file from the combo update.
Get
Pacifist
Pacifist is a utility that can extract files from a .dmg folder.
Open Pacifist after you install it. In the Pacifist window click Open Package.
Navigate to the MacOS XUpdCombo10.6.7.pkg. Select that then click Open.
In the next window click the gray disclosure triangle where you see: Contents of manual
Then click the triangle next to: Contents of MacOSXUpdCombo10.6.7.pkd
Then click Applications, then right or control click: App Store.app
Then click: Install to Default Location. That will reinstall the MAS to your Applications folder.
Carolyn 🙂