If you add a custom domain later you'll also have to update your kit's domains to include that. If you do add a domain and have already added an ID then the updated domain list can be found next to it in the DPG admin interface.
Once you've created the kit you'll be given an ID which you can use in DPG.
Next in the DPG admin interface on your website screen make sure you are using at least standardedit mode.
You can then add your ID in the Typekit section.
With the kit added you'll be able to use any of the fonts that you have included in the DPG visual design editor, all you'll need to know is the CSS font name which can be found in your Typekit account.
From there using the font is as easy as copying and pasting the CSS font name into DPG.