I need help with syncing a movie from my itunes to my itouch.?

Okay I have an ipod touch first generation, yeah I know so old. Well I recently bought paranormal activity 2 tickets on fandango and they gave me a free paranormal activity 1 movie download from itunes my mom forwarded me the email I downloaded it onto my itunes but, now it says it's not authorized on my computer so it wont go on my ipod touch. Help?? I've never put a movie on my ipod before.