Can I interrupt an endless loop in installation?

Question

Hi, I'm new to Macs and new to Garageband, having messed around on Ataris and PCs for a few years. 
Amazed how easy it is to get started.
I laid down my piano part.
Then noted that there might be an update to Garageband so clicked to install it.
OK let's put some strings to back the piano.  O dear, "You have already initiated the installation of additional content for GarageBand via Software Update. For additional information please open the 'Software Update' application." In downloads I find "Garageband6.0.5Update.dmg".
OK, lets open that.  Oh dear, "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 there isn't.
How do I get out of the endless loop?

Answer 1

For some posters it helped to reinstall GarageBand -
Quit GarageBand and move the application to the trash (but don't empty it).
Log off and on again, to make sure, GarageBand really terminates after quitting.
Sign into the AppStore and now you should see "Install" besides GarageBand. Reinstall.
Make sure, you are still signed into the AppStore, and then try the "update later" method above.
If that does still not work, you may need to do the "full monty" and deinstall completely, see Dagon's post:
https://discussions.apple.com/thread/3039094?start=0&tstart=0
Regards
Léonie