This page covers the mounting and retention of the rocket motor. For information on the transfer of force from the motor to the rocket (which sometimes can be down through the motor mount), see the Thrust Plate page.
The motor must be constrained in all 3 axis of movement inside the rocket. Usually, this is accomplished with a tube that stops the motor from moving side-to-side in the rocket, a thrust plate that stops the motor from moving up, and a retainer that stops the motor from moving down.