There's no 'buying option' on the mpex website because it trading is done via irc.  If you're needing to ask all these questions to work out how mpex works I would venture to suggest using a passthrough might be a better option for you.  There's also the matter of the 30btc fee (or 25 if you hook up with one of the existing mpex members who would get a sign-up bonus) so unless you're planning on investing heavily or doing a lot of trading mpex is probably not the solution for you.