iPod touch My apps wont download onto my ipod.

[Eee- cieee. :)]

I have downloaded the apps i wanted from the istore. This was successfull. But when i go to sync it onto my ipod a box pops up sayin:
"This application..... was not installed on the Ipod touch[/SIZ because you are not authorized for it on this computer. I have tried everything i can. Has anyone else had this problem? And can anyone help me? I would like some apps on my ipod soon. Thankyou.

 

[Tallest Skil]

Authorize the computer under the Menu Bar option called "Store".