It is sort of difficult. Basically you have to login first. Then "challenge" will appear on the nav bar. You may have to e-mail Nik your mods and he'll punch it in for you.
On Saturday, you get the transponder for the whole day. They are trying to get enough cars to have a "challenge" session only. 2 years ago we just took the fastest time from any of the sessions. I actually only ran one session because I thought my transmission was toast but it turned to just be a leaky seal.