Brown doesn't admit people to the 4 year MD program in a usual way. It admits people to a 7 or 8 years combined BS/MD degree. So you apply to that when you get out of high school.
Another way to get to Brown is to apply to Dartmouth. Once you get an interview, you can select to apply for the competitive Dartmouth-Brown program. You'll do your first two years at Dartmouth, then the last 2 years of clinical rotation at Brown.
The third way, of course, is to apply to the MD/PhD program.