Go to iTunes menu bar/Store and click authorize this computer. You are allowed 5 devices, if you have maxed out you will need to deauthorize a device.
You can do thus by going to another device and clicking deauthothorize this device.
You can also go to iTunes Store in the left column, clic thatthen click on your account (Usuually toy email) in the toof the iTuness page (you will have to enter you Apple ID)
Under Itunes in the the clound you can click manage devices.