Multiplayer is accessed from the cat captain in the village, he should be right under the item shop I believe. Not really sure how it all works, I know no one around with a 3DS, much less Monster Hunter. I ran across someone at my school today who played it, so not I just have to hunt them down... If all else fails there's likely a video or tutorial somewhere.

Anyway, picked the game up the day it dropped, happened to be the day that I left on vacation, so not much was done there. Still, love it so far, and it's just kept on getting better!