Sometimes enemy planes fly away from you, and you can't catch up. That kinda sucks, but I found a solution: Turn away. If everything is alright your opponent will turn towards you. So flying away for like 5 seconds will do and if you turn back the enemy will fly towards you. And thus makes it easier to complete the mission.
Go to the weapons loadout screen and type in I AM THE ACE
Go to the campaign menu and left click (some say right click) on the microphone at the left of the screen then type in idaho. Now you should be able to access any mission thru a menu that should appear.