How to use 802.1X authentication with Edimax EW-7811Un USB Wi-Fii adapter?
You do all the work in the "Network" System Preference if memory serves.

I don't have a 10.6 box handy to check but under 10.8 and 10.9 you first add the correct certificate to your keychain then when you select any of the "Enterprise" security options and EAP-TLS as the mode you can tell it to use the correct certificate and the username.

I remember having to  write the instructions for doing this on 10.6.8 a few years back but can't find them. The process was highly similar though.