Hi Mike:
There a couple of settings you need to enable.
The steps per degree should be a simple calculation based upon the ratio of the rotary table, the reduction ratio from motor to rotary table input shaft (if any), and the steps per revolution of the motor, which is usually 200 ppr for stepping motors. Also, if you are using micro-steps, this must be accounted for.
You did not say what the rotary table ratio was, and if there is any motor reduction between motor and rotary input pinion gear.
For the rotary table to act like a normal axis, go to the "config" menu, and scroll down to the heading "Toolpath."
The upper right box is labeled "Rotations." Check the axis of rotation, usually the X axis; check the A-Rotations Enabled, and check the "Use Radius for Feedrate."
Click OK, and scroll down to the bottom of the config menu, and click on "Save Settings."
Now open the "Settings" menu. Check the appropriate boxes for Axis you will be using. The X,Y, Z, and A should be black, and the B,C lit green.
The heading in the upper right corner is for "Rotation Radius." Since you checked the "Use Radius for Feedrate" in the Toolpath menu, here is where the part Radius is entered to make the axis's move in sync. Enter some general value, like 2.00 if you are using "Inches". If you are doing a real job, you should enter the actual part radius.
As before, go to the config menu and "Save your new settings.
Under "Config", open the "General Config..." page. On the left side of page is a box "Angular Properties." Check "A Axis is Angular." Click OK, and go to bottom of menu and "Save Settings."
Now close Mach3, and restart. Your new settings should all be enabled, and the rotary axis will act like another axis.
You must make sure the correct motor steps for 1 degree of movement. Usually, you can set the A axis Velocity and acceleration quite a lot higher than the X,Y, or Z axis.
Do a test with the MDI line. Set X to a move of something like 5", and A to about 40 degrees; G1 X5 A40 F25, and see how the two axis move. It should work in sync just like an X and Y motion.
If a command to move 40 degrees does not come out "Exact", you have made an error in the steps per degree calculation.
Also, this is all assuming you have the motor settings correct for steps per, velocity, and acceleration. This is all trial and error, and listening to the motor sound.
Hope this helps.
John