What is the little plus sign on some apps in the app store?

Why can't I update my apps when logged into to the App Store in OS X?

• When I try to update an app in the updates section, It gives me this error message: "To update this application, sign in to the account you used to purchase it." But I am logged into the account that I bought the app with. I only have one account and am the only user. I tried relogging into the app store, but no luck. I also tried reinstalling Lion, but didn't work either. Is there another way to reset my apps/app store?

• Answer:

  You may have installed an application purchased on another Mac. Applications contain a receipt within the file that tells the App Store what account was used to buy the app. If if is a legitimate app you can just delete the receipt and it'll update just fine. You can also try to repurchase the app, since you have already paid for it you won't be charged again. To delete the receipt, locate the app in Finder, do a ctrl+click, right click or double tap on the trackpad and select "Show Package Contents" then navigate to Contents>_MASReceipt and delete the file that is in there. Drag it to the Trash and empty the trash, just in case. Relaunch the App Store and the problem should be gone. Don't forget to quit the app before modifying its files. Good luck.