hmm... if it was already registered with Steam you shouldn't need to use it again. At least I haven't had to with other games. I don't actually have TF2.
Did you buy the game on disc or through the Steam store? If its through the store you won't need it again. You should be able to just re-download it if needed when you login. If it was on The Orange Box disc you would just need the key for it. You may want to contact their support if its not letting you play.
I tried contacting Support and got no reply as yet, that was 4 days ago.
I bought it through Steam, but I tried to install it using a Disc. I was told that I could get my CD Key from Steam, but I can not find it anywhere. My ISP nukes my download spead after 2g, so it will take me about a week to download 9g.
Ouch that is harsh! They should have contacted you by now. The cd key should be the same as what you use when you install it from disc. The id number you have to punch in. You should be able to put that in and have it work.
