I answered you on Facebook and am answering you here.
If you want to keep it simple, you need a relay board with dry contacts:
6 Channel 24VDC Relay Board PLC DIN Rail Mounting NPN
http://r.ebay.com/addtr0.
Connect it to outputs 3,4,5,6,7,8.
You need to put 24VDC from Acorn's power supply into the COM terminal on the relay board.
There are two COM terminals next to -1, -2, -3, -4, -5, -6. Either one of them is fine to connect Acorn +24VDC power to.
The -1, -2, -3, -4, -5, -6 go to Acorn OC 3, 4, 5, 6, 7, 8. Giving you dry contacts controlled by those outputs.
Now use the relay dry contacts between your VFD's 24VDC and FWD/REV/RESET Terminals. (see below)
You have to do this because your VFD does not have the option of using COM/GND OR 24V.
You can't tie the 24V of your VFD to Acorn common.
In the wizard
Program OC6 to RESET (Connet the Common of the relay 6 to VFD 24VDC, connect the NO terminal of the relay to VFD Terminal 9)
Program OC5 to Spin Fwd (Connect the common of relay 5 to VFD 24VDC. connect the NO terminal of the relay to VFD Terminal 10)
Program OC4 to Spin Rev (Connect the common of relay 4 to VFD 24VDC, connect the NO terminal of the relay to VFD Terminal 11)
Take VFD terminal 15 to Acorn COM
Take VFD terminal 16 to a spare input on Acorn. (Program this input Spindle OK in the Wizard)
Test that you got the relays wired up right and working. You can turn them on and off manually via the manual spindle control on the virtual jog panel. The relay board is nice because it has an LED that let's you know when its on.
My best guess to connect Acorn H8 AN GND to VFD terminal 1, then connect Acorn H8 AN OUT to VFD Terminal 2.
BUT BEFORE YOU DO THAT,
make sure you tested the outputs first and they are closing the proper terminals on the VFD.
Connect a 1.5V or 3V or 9V battery to those VFD terminals. Terminal 1(negative/COM) and Terminal 2 (positive) then turn on the output. If the spindle motor runs when the battery is connected and stops when disconnected, you can connect those terminals to Acorn.
Of course you need to program the VFD appropriately.
Take it slowly and one step at a time. You should be OK