As far as I'm aware all you have to do is release your map on their forums as if you would here. They have a halo forge forum. Also if they choose your map they will choose it for their official gametypes which they use at tournaments and much more, not only the MLG playlist, which would only get updated a month after.
You post it up on the MLGpro.com forge forum, then you actually stick with it and constantly update, test like a madman, and overall stay attached to your project if you really want it to be good enough. After last season, only 6 maps were even selected to be eligible for MLG pro circuit, and out of those 6, only one made it through the excruciating testing process alive, and now we've got Lockdown Ball to show for it. Oh and the map should be good too, but I thought that was a given once you've done the above.