can't download U2 album (this computer is already associated with an Apple ID)

Title pretty much sums it up. I can see the recently released, free-of-charge U2 album in my purchases, but when I click on the cloud icon I get the message that "This computer is already associated with an Apple ID" which is not at all revealing of just what the problem is. Then there's this bit below:

"You can download past purchases on this computer with just one Apple ID every 90 days. This computer can be used with a different Apple ID in 2 days."

What the heck does THAT mean?