Vince -
As Peter said - that depends on your own particular motor control system. The Digispeed provide a voltage signal (usually 0 - 10v) to control the speed, as you seem to have sorted out.
The direction depends on what your own VFD demands.
Mine is an Omron and (as well as the voltage input for the speed) it reqiures two sets of wires connecting to give CW or CCW rotation. You can connect none, or either pair, but not both. Both is an error and will stop the Omron.
The standard instruction for CW rotation in GCode is M3, for CCW is M4 and off is M5. The M3 and M4 signal, as you say uses two output pins from Mach. I have used pins 8 and 9 to drive two 5v relays. (I actually made up a board with four relays, all driven by a Darlington array, so I could do the coolant as well) Darlington arrays are cheap (less than £1 for an eight unit array) and they run directly from your computer output. They are rated at 1/2 amp for each output, so plenty of meat there.
The advantage of using relays is that it means that your VFD is completely electrically isolated from you computer and other electronics, and as Peter says in his blurb with the Digispeed, sometimes the voltages are not to be trusted.
On the Mach screens, you need to untick the "disable spindle relays box" on the Config/Ports and Pins/Spindle Output page and assign M3 and M4 to outputs (usually 1 and 2.) On the Config/Ports and Pins/Outputs page, you then need to assign Outpus 1 and 2 to pin numbers, and enable them. Peter seems to have coped with this in the Digispeed, so you will have to read the blurb to see if they are active low or not.
I personally have used the PWM output from the Mach to control the Digispeed. My Digispeed could be altered to suit either. This comes out of Mach 3 on a pin specified on the "Step" output of the spindle motor drive. (The Dir" pin is ignored).
So - in a word, there is no simple way - it depends on what your VFD requires. Have a look at that first, and work your way back from there.