Enable the advanced editor functions ( top menu ).

Right click on the engine and click "add".

Select the smoke component.

You'll have to assign smoke to a virtual servo.

Typically everyone uses the throttle so the smoke density is controlled by the throttle mimicing real life.

Poke around in the editor. You cannot do any real damage to the default models.

For the user created aircraft, just save your edits under a new name.

The sim is so accurate that you can also do predictive modeling, where you can effect changes in a plane, then try doing the same to the real thing if you like the effects.

A lot of guys use to to work out the kinks with their scratch builds.