You have to go through Gaemespy Arcade and find the server dedicated to that mission.
The problem is, the Army only dedicated about 3 servers to that training mission, and there are about 300,000 people trying to complete it. Only 48 can do so at any one time. And before you can advance to the next mission, you have to complete that one on those servers. Not only that, but you have to complete it with a satisfactory score and save/upload the results.
Bad move on the Army's part for them not giving you the option to make your own server.