A few days ago Microsoft released a KB article which explains how to add a product key to an evaluation version of SCOM 2012. It is a simple procedure. You can find the KB article here:
How to add a product key to an eval version of System Center 2012 Operations Manager
You can check if you are using an eval version of the product by opening the SCOM Console and going to Help -> About.
Good luck with that!
Bob Cornelissen