I got a new MacBook Air. I registered with iCloud purchased iTunes Match and now my whole iTunes library (originally on my iMac) shows up in iTunes on the MacBook Air, but it's all grayed out and nothing will play. What haven't I done?
This is likely because iTunes Match is enabled on your MacBook Air under the same Apple ID your purchased it on and configured it for on your iMac. When you configure iTunes Match on a second machine, it puts the entire iCloud library in iTunes grayed out with the option to download. There should be a small download button in line with your songs that will allow you to download the matched songs: