Wow, that is so hard I didn't think it was being seriously considered. They're going to send a Mars surface-to-orbit launcher as payload in one piece, without fuel, and then produce >30 tons of rocket fuel on Mars? My hat's off to them.
Musk hasn't discussed the exact details of his plan, but it seems like the unmanned missions in 2018, 2020, 2022, and possibly 2024 (if there's no manned mission that year) are meant to establish a rudimentary fuel production infrastructure and start producing fuel in anticipation of a manned mission.
NASA, I believe, is currently trying to solve the problem with more efficient engines for interplanetary travel. Ion drives seem to be the preferred method, but there is still some research going on in nuclear thermal rockets.
