Quit the Mac App Store if it's open. From the menu bar App Store / Quit App Store. Or Command + Q on your keyboard.
Launch iTunes then from the iTunes menu bar click iTunes / Preferences then select the Advanced tab.
Click: Reset warnings and Reset cache. Now from the iTunes menu bar click Store / Sign Out.
Try the Mac App Store.
If you still have problems, follow the paths here. ~ (Tilde) represents your Home folder.
~/Library/Caches/com.apple.appstore/Cache.db. Move the Cache.db file to the Trash.
~/Library/Preferences. Move the com.apple.appstore.plist and com.apple.storeagent.plist files to the Trash.
~/Library/Cookies. Move the com.apple.appstore.plist and Cookies.plist files to the Trash.
Restart your Mac. Try the MAS.
If nothing has helped reinstall the MAS by downloading and installing the v10.6.6 combo update available here.