On the 3 axis version of the blue TB6560 board pins 15, 16, 17 are not used and are availlable. You can tap into them on the bottom of the board, at the 25 pin edge connector. Just make sure you understand the pin numbering top and bottom.
In MACH3 if you go to the Spindle setup tab and select "Use spindle motor output". Now go to the motor outputs and you will notice spindle Enable is now active. You can now set the output to a free pin on port '1' .
Once this is done you need to wire the pin you chose to the 25 pin edge connecor on bottom of the card. Route this wire to your PWM board. Now slder a proper ground wire back to the 25 pin edge connector of the TB6560 board (one of pins 18 thru 25). Setup MACH3 for the PWM signal.
I suspect this should work.