I believe you have to type in your user name and password (same one that you registered Impulse with, at least it was for me) then either login or create account (sorry, don't remember what it says exactly). Once that is done, under manage account, you should be able to create a profile, for some reason, it doesn't take the profile you use for single-player. Once you've created a profile, just select it and hit use profile (I think) and you should be able to look for games, host game, the whole multi-player thing.
Sorry if this is a bit vague, but it's been a while since I signed up for multi-player, but I do remember it wasn't terribly intuitive.
