Go to any online arena including custom arenas, click on Staged Missions, then click on Select Mission, check the box "show missions for all terrains" above the mission listing. Click on any mission, then click on download mission. Offline you have to choose the terrain the mission was written for at the start up screen. Good luck and let me know if you have any more questions. I have a list showing available missions posted in a thread a few threads down from this one, but it has not been updated in months.
