Order ID 8701

I just purchased and this doesn't appear to be working as expected. I create a custom url with the following information:
SEF URL: testaffiliate
Old Non-SEF Url: index.php?affiliate_id=83

What this should do is set a session variable in the backend that we use during sign up but it doesn't work. If I go to my site (i.e. www.example.com/index.php?affiliate_id=83) it works fine but not if I go to www.example.com/testaffilite.

How can I get this working?