The community is also a great place to check out. Try these places for help from other users:
Please review the Third-Party Game Support topic to determine whether your question should be directed to the game developer or to Steam Support. Sending a support request to the wrong party may result in a delay in processing your request.
Retail CD-Keys for this game cannot be registered through Steam. If you wish to create a shortcut to this game under your Library, instructions can be found here:
Customers who purchase a package containing both Doom 3 and Doom3 Resurrection of Evil may run into an issue where Doom 3 prompts them for a CD-key. If this happens to you, please launch Doom 3 Resurrection of Evil once. That should resolve the CD-key issue with Doom 3.