From the beginning, we've only allowed registration through Steam. That's because our website, and especially our store, runs off information obtained by Steam to integrate our website into our game servers.
We knew that registration was not always possible, and because of that, we've now implemented manual registration. You'll now see an option to input your Steam ID manually during registration.
We still recommend using Steam to register, but if that's not an option, manual registration will do just fine. Just be sure you input your Steam ID correctly so our system can accurately identify your in-game should you use our store.