how do I add a managed device to my Itunes account?

Set up my account on a new computer & authorized the computer, however there are 5 songs that are not allowed to sync on my Iphone now saying "you are not authorized to play it on this computer". Songs were purchased under a different account which I had originally used but no longer have access to. My current account Managed Devices do not show my new computer, even though it shows as an authorized computer. Whats going on here?