I just downloaded a few free apps from the App Store to my itunes but unable to sync to my iphone. Get an error, 'This app cannot be installed on your iphone as you dont have the necessary authorisation on this computer'

Can anyone help as to what the problem could be and how to rectify it? Thank you in advance.

Cheers.

PS: Also, just upgraded to 2.1 using CFW but unable to sync my video movies with the iphone. Can sync everything else though. Please help.