1) Go into config/ports&pins to enable A
2)Go to config/toolpath and check a-rotations enabled and check your corresponding axis of rotation.
3) Go to config/motor tuning and set steps per (this will be steps per 360 degrees of rotation) and velocity (this will be revolutions per minute)
Your program run (Alt-1) screen should show x, y, z, and 4. Hitting tab key should fly out the MPG and you should be able to see a 4th pair of buttons for jogging the 4th axis.
I've been working on this as I will begin building a separate machine for turning using a CA Harborfreight lathe. The advantage there is to be able to make an extended bed using steel angle. I'll mount the whole thing to an i-beam for rigidity and feed my router horizontally. It should be a fun build.
Bob