At the top, under the paradox banner is the menu bar - with News, Forum, My Games, Blogs ... etc.
The "Forum" is currently highlighted as you are in the forum.
click on the my games link, then from the drop down select the game you wish to register. Go to Steam, games list, right click on the game of choice to register - select "View Game CD Key" and click copy to clipboard.
Then back to the web page - paste the data into the appropriate text box, and click submit - voila, should be done (with a short wait).
Thank you! Worked