You don't need a license with CM6; you just need to make sure you have enough DLUs. If it were CallManager Express or the Windows version of CallManager you would need a license. However, since version 5 CallManager tracks it's licensing on the server via DLUs. To see your licensing, go to System --> Licensing --> License Unit Report.
If you do not have 3 free DLUs for the 7937, you'll have to purchase some more.