I have my preorder code for my Downloadable Content but I cannot see how to activate it.
To activate your preorder code open your Steam client and select '+Add a game' from the bottom left of the screen. From the drop down list select 'Activate a Product on Steam' and then enter your code exactly as it is printed. Ensure it is entered in upper case and that you include the dashes also.
If you have not done so already, ensure that you activate and install Civilization V via the code on the back of your manual first. If you do not the DLC code will not work.