I had exactly the same problem as you. I bought a 2007 Trek 520, same Schwalbe Marathons, and the same thing happend. Just could not get the last bit of tire over the rim. I couldn't roll it over the rim, and the tire levers kept popping off. Get the tire over one end of the rim, then it would pop off the other end of the rim.
After a week of struggling with it, my hands had no strength left, and I took it to the bike shop. They told me its easy, just roll it over the rim. Then they tried and couldn't get it on. They used some soap or something, and it took 3 guys an hour to wrestle both tires on. Thier hands were hurting so bad after they got the tires on, and I knew if I get a flat in the middle of nowhere, I'm in trouble.