Why is my iTunes library in iCloud grayed out?
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:



Good luck.