To access the doorway form settings you will navigate to the Lead Capture section. This can be found by selecting Website on the dashboard menu, and then Lead Capture at the top of the page.
Individual Page Doorway Form Settings
Under the trigger settings you can find individual settings for each page for lead capture and doorway forms. To enable doorway forms on a page, click the page you are wanting to add a doorway form to, then you can select the form and contact group that it will be associated with from the drop-down menus on the right.