Is there some syntax or rules should I follow?

Description of my Operating system: Ubuntu 9.10 Version 2.1.10. tar.gz of freeradius in the user file and remove some comments to add users, I've put everything you need, just do not know how to declare the MAC addresses to access let me alone with them, apart from the IP Is there some syntax or rules should I follow? What I can do about it? I hope to be concrete and someone can help me