It's unintuitively under the "Add/Remove Members" tab on the "Edit Network" page. Once you click that, the radio buttons to select "Gateway" or "Member" (or Hub or Spoke) appear. It's in the FAQ, but of course you have to know what you're looking for.
Almost two years later...you provided my answer. I've actually had Hamachi running for over a year, but it is so reliable, that I never have to touch it. Only when I had to re-install Server 2003 (from scratch!) did I remember how unintuitive the setup process is. Now I have to tell 25 people to reconnect to the server. Oh well, better than telling them they can't connect to the server, eh?
I suppose you get what you pay for! Can't see paying for this service when it is difficult to establish a gateway network. Can't get a Windows 7 machine on a LAN?homegroup to be seen by any number of clients. All indicate that the gateway is offline, "Domain disabled" in settings, even though the Users Guide states gateway is not possible when the gateway machine is a member of a Domain?!?!?!?! What am I missing having followed the Lessons Learned" post well written at the top of this thread. Thx for the work on this BTW. Wish support was as clear as the suer who posted lessons learned!