The game can be downloaded via Impulse, there is no link to download it in a browser. Once you have signed in on Impulse with your forum account, click the (?) in the corner to access the registration screen where you can add the game to your account. Once you have added it, go to the My Games page where you can select and install it.

Do please note that you need to have the base game registered on the same account, and update to the latest version before you install Entrenchment.