Populate Missions

When a Staged mission is loaded, you may join by going to Staged Missions=>Populate Mission. Under the Not Assigned tab you will click on your name and then double click the plane you wish to fly. You can also click on the side (Axis or Allies) you wish to fly and you will be randomly assigned a slot when the mission briefs. 

If you wish to leave the mission and free fly, you will need to go back to the Populate Mission screen, click your name and then click Unassign.