How do I become a volunteer for May @ Mac?

It may be a little late now, because it's so close to Saturday. But maybe try for Welcome Day in August? It also depends on the position, e.g. residence tours are done by CAs and IRC members, tours are done by hired people for the summer (by the Student Recruitment & Campus Tours office). You should contact the Student Recruitment & Campus Tours to see if they have any volunteer openings though, and they would definitely point you in the right direction.