If you still need help...
Try here > Mac App Store: "An unknown error occurred (100)" when purchasing
If that doesn't help, open the Finder. From the Finder menu bar click Go > Go to Folder
Type this exactly as you see it here;
~/Library/Caches/com.apple.appstore/Cache.db
Click Go
Move the Cache.db file to the Trash.
Now here:
~/Library/Preferences/
Click Go
Move these files from the Preferences folder to the Trash.
com.apple.appstore.plist
com.apple.storeagent.pllist
Now here:
~/Library/Cookies
Click Go
Move the com.apple.appstore.plist file from the Cookies folder to the Trash.
Empty the Trash, try the App Store.