Aaronr,
Create a custom arena and a pop up box will ask for the terrain among other things. The pull down menu will have all the available terrains listed. Choose one you want and when you create the arena it will download to your computer. To see the missions, once you are in the arena or the main arena, click on staged missions, select mission and check the box show missions for all terrains and you will see all the missions on the list and be able to download them. Let me know if you have any problems.