You cannot take off with the Mistel so if that was what you were trying, it's a no-can-do.
Here's how to set up the Mistel in an airstart:
Make two waypoints for the FW-190 (Mistel).
Make two waypoints for the Ju-88 (Mistel).
"Set" the 190 waypoints to the Ju-88 waypoints in the same way you would have the 190 escort bombers.
Aim the 190 at your intended target.
Make sure you have "Detach" bound to a key or HOTAS button. The game does not consider it a bomb, it considers it a drop tank. (And what a drop "tank"!
Detach when the target is centered in your reticle.
Get the heck away before the Ju-88 hits something.
Aiming the Mistel takes practice but even the misses are spectacular!