How do I move purchases from one library to another?

From the same Apple ID I have two libraries, one on a PC and one on a Mac. I use the Mac for music and the PC for other stuff. I bought an album (music) and downloaded it to the PC library accidentally. How can I move it to the Mac? The Mac tells me everything purchased has been downloaded, which it has, just not where I want it. If I try to sync my ipod with the PC it tells me either I can't connect the same device to more than one library or warns me that I'll delete all the music already on it and just sync the one tune. Many thanks in anticipation.