A few other things to try when you are not seeing updates in the App store.
1. Quit the App Store and then delete Motion from your system (drag to the trash and empty). When you relaunch, go to your purchases and it should have the “Install” button available. This will grab the latest version of the application. *Note: If you need to keep an older version, compress it and move it someplace outside of the Applications folder for storage.
2. Make sure you don’t have a copy of Motion on another mounted drive/partition. If you do compress it or unmount the drive before checking for the update with the App Store.