The on-line key should be printed on the back of your Manual.
If you don't have one, I believe there were some copies released in the US without a code, and Simbin had posted a key for people to use. (I can't find it now though??)
Have a read of this thread at RSC:
(Solved) CDN/US release of GTL No online cd key !!?? - RaceSimCentral Forums
Also maybe the FAQ might help:
Frequently Asked Questions (FAQ) - RaceSimCentral Forums