I installed a copy of Civilization 4 that I have bought and when I installed it, it didn't ask for a serial. But in Steam, I can only see an option that requires you to add the serial number. I checked the box, manual, etc. but I didn't find any serial number. How can I add my Civilization 4 game to my steam account?
Not all games that are released through Steam support adding the version not bought through Steam to the Steam library.
This is the up-to-date list of games that support registering them in Steam. As you can see, it doesn't include Civilization IV.
However, remember you can still add the game as a "non-Steam" game. This gives you the benefit of the Steam overlay, but does not make the game auto-update and does not allow you to download the game from Steam.