computer not "authorized" for downloading purchases

I recently purchased a movie on itunes on my phone and when I try to download it on my computer a message pops up that says "Your computer is not authorized. you must authorize this computer from the account menu before you can download previous purchases" this is on a 5 year old macbook air which does not have the newest update, if that will help at all. I have already looked in and messed with the settings for 30 minutes and it has not helped.