Did you get an email with your serial #? (not the order number).

Did you use paypal? If you did, is your paypal email different from your Impulse account email? The registration is attached to the email you used when you bought it, so if it's different, you'll have to contact support@stardock.com for a merge.

If it's the same, go to the Impulse orb button in the top left and Add Registration, see if Entrenchment shows up in the list. If it doesn't and you got the email, try to manually add the registration for it.

And of course.. restart Impulse in case it's stubborn and doesn't want to update your My Games tab with Entrenchment.