You have updates available on another account
I had a similar issue when I first got my Mac.  The first thing you want to try is going to the "Purchased" tab in the App Store and seeing if it gives you the option to "accept" the iLife apps at the top of that page.  If not, then you should contact the App Store support team using this website: http://www.apple.com/support/mac/app-store/contact/ .  They will most likely email you back asking for your serial number to confirm that you are eligible for the apps.  For me, they validated my serial, verified that I was eligible for the apps, and issued me redemption codes for all of the iLife apps.  After that, no problems.  If they issue you codes, make sure you uninstall the three iLife apps through Launchpad before using the codes and redownloading them.