Just grab Impulse, log in, and it'll show up in My Games when you have "Show pre-release versions" checked in the blue orb menu in the top left. Note that you'll need to have vanilla Sins updated to 1.12 *before* you download Entrenchment, otherwise it will be all screwy and crashy.

Check your registrations by clicking the ? in the top right - it should've been done automatically. Does Entrenchment show up there in the list?

No problem Quite odd, though, since as I said it's meant to be automatic when your order is processed