OK. If on the MAC you mean iMac (?) then:
Purchases on that computer won't show up on another computer automatically. You need to go authorise the computer as
flynz4 suggests, and then you'll be able to access content from the iMac via your MBA through
Homesharing if you're on the same local network.
If you're out and about and want access to your iTunes library, then you'll need to
copy your TV shows etc over to the MBA.
I'm based in the UK so not sure exactly how the
iTunes in the cloud stuff works, but looks like you can/ will soon be able to redownload purchases from the iTunes store onto different computers.
Have I got the gist of what you were asking for? Provide more details if not!