Actually the Maclays are Equitation, not hunters. The Maclay Finals are one of a few Finals that are nicknamed the "Big Eq" Finals, set at 3"6 for juniors (17 and under). They're pretty much the top of the Equitation world and was recently made more popular by a show Animal Planet put on a few years ago called "Horse Power: Road ot the Maclays" or something. IT's kind of a fun series you can find on youtube! It's extremely difficult and very prestigious. It's almost impossible to even qualify without a top trainer, top horse, and lots of expensive horse show experience.
To qualify you need to first compete at at a regional qualifier held sometime in the fall. I think they take like, the top 20 or something. Once they have their list of 150 of the top Eq riders in the country there's the "Finals" held at the National Horse show, which is usually at the end of Oct or beg of Nov. It used to be at Madison Square Gardens but has been at the Syracuse Invitational the past few years. I'm not sure where it will be next year, I hear the SI sadly will be no longer.
At finals there's a flat phase and a jumping phase, then so many riders get called back for another jumping phase. The 2nd jumping phase is usually a pretty difficult test. From there they pick a winner. Most winners go to be grand prix riders, even olympians.