With the Harbour Master DLC you can enhance Port Royale 3 with six completely new building types, several decoration objects for your cities and with an economic adviser, who will give you useful hints. In addition you will now have more control over the defense of your cities and you will be able to build ships on your own!
Open the Steam client, login and locate the Games tab on the top menu. Click this and then select “Activate a Product on Steam…”
Enter your unique code that's displayed on the 'Orders' page of CDKeys.com and follow the prompts to activate.
Your game is now viewable in the “Library” tab and is available to download/install when you’re ready. You can find additional support for the Steam client here: https://help.steampowered.com/en/wizard/HelpWithSteam