In what file are enlisted to users?

Hello friends my name´s Jesus I have a problem, I am using freeradius on my virual machine to the ubuntu version 9.10 and want to add clients for my freeradius detect the mac address that I enter. The question is: Is there any easy way to add clients? Yo I have installed: The virtualbox version 3.2.10 Ubuntu version 9.10 Freeradius version 1.2.1910