I bought this game years ago (Physical DVD - Ultimate Edition in 2009) and just noticed it was on Steam so I checked my old emails and re-logged onto my Stardock account, redownloaded and installed it and everything is fine and it works.

But how do I acivate it as a game on Steam?

There is an option to add non Steam games and thats works but all that really does is create a link in Steam to launch the game

There is an option to activate a game by entering the product key but the product key shown on my Stardock account for the game does not work in Steam.

Am I missing something or is there something else I need to do?