I'm installed the game using the code on the box but have a pre-order code for the Tribal Gear Pack (didn't get round to installing earlier because my gaming computer was bust). The instructions in the email say:

1) Log in to STEAM and go to Games, then Activate product.
2) Enter your code and follow the on-screen instructions.

I think I must have missed the point where I was supposed to enter the TGP code, how can I do so now?

On Steam, on the bottom right, you should see an "Add a game" option. Click it, and then select "Activate a Product on Steam."

Did as instructed and the pack downloaded automatically,

Thanks for your help, much appreciated.