When you launch the game from the 'Play' button in Steam, you should see the following:

On the Stardock Account registration window that comes up, it should say 'Sins of a Solar Empire - Trinity', just like is highlighted with the red box.

If it says anything other than that, (Sins of a Solar Empire - Diplomacy or Sins of a Solar Empire - Entrenchment) please do the following:

• Navigate to where the game is installed (by default C:\Program Files\Steam\steamapps\common\Sins of a Solar Empire Trinity)
• Run Sins Trinity.exe
• Click 'Play Trinity'

You should then see the window from the screenshot above. Enter in your account information and serial number and then click 'Register'. If you get an error or it does not show Trinity in the activation dialog like the image above, try running one of the other executables (Sins of a Solar Empire.exe, Sins of a Solar Empire Entrenchment.exe, Sins of a Solar Empire Diplomacy.exe).

Once this has been done, it will activate the base game and all of the expansions and you should not see the registration dialog again. You would then be able to launch the game normally through Steam going forward.