to create an iMix, you just make a playlist on the side of your iTunes, then drag songs to it from either the store or from your library. You don't have to own the song to put it in your iMix. Then, when you've got the songs you want in your playlist, highlight the playlist and then go to the menu (above) and under "File", there is an option to "Create an iMix". Click on that and follow the directions.

It's really easy.