Welcome aboard. 8)
I think the answer to your question is that there is no Marine Corps ROTC because the Marine Corps is part of the Navy and therefore you would have to enroll in Navy ROTC.
It's been more than 40 years since I was in ROTC, so maybe you should get the straight poop from others.
"It doesn't take a hero to order men into battle. It takes a hero to be one of those men who goes into battle." - Norman Schwarskopf, Commander of Desert Storm Operations