It seems like one of the most common problems people are having with the Foundry so far is the "First map transition to a custom map has to specify a door" issue. Just for clarification on what this "door" is, when you create a mission and someone accepts it, they arent automatically transported to your map. They have to go somewhere in the existing game world to enter into your first mission map, and whatever place they have to go to is the "door" into your mission.
So what you have to do is select a "door" that leads the player into your first mission map. To do so, go to your Story tab and find this box and click on the button I have circled. That will bring up this box, where you actually select the door. Select the ground or space map you want, and then select the NPC/space system you want to be the doorway to your mission. If you create a mission with more than one map, you'll need to repeat that process when going from one map to the next. Hopefully this help a little