Trying to install Logic Pro

Can anyone help me. I bought Logic Pro 9 from the App Store a few days ago. It would not download because I did not have enough disk space. I deleted the icon thinking I can re-download it from the App Store on to my external hard drive which is formatted for mac, but the "INSTALL" button in the App Store is greyed out. How do I re-install Logic Pro then? I'm running OS X 10.7.3 on the Macbook Air.

Thanks.