have downloaded the newest version of garageband but can't open it: getting the following message:

I have Macbook Air that had garageband pre-installed. I am trying to upgrade to the latest version. I get the following message when i try to execute the downloaded file:

The version of GarageBand installed on this Mac must be updated through the Mac App Store. Check the Mac App Store to see if an update is available.

And I did it through the mac apps store....so i don't get it!