Motion wont install from app store after update

Went to install the latest update to Motion today. Towards the end of the update I got an error. I re-tried the update but the error keeps occuring. I deleted Motion and went to my purchases to reinstall a fresh version. Everytime I click "install" I get an error telling me I need to install Motion from the "Purchases" section (which I'm already in). I need to get this working by morning. Any ideas how I can fix this?