Ok, I got this all sorted out. If you make your own RJ12 to DB9 connector you need to make a straight thru connection. This isn't clear in the Automation Direct manual because it shows RXD going to TXD which would be a crossover cable. The pin connections are as follows:
RJ12 DB9
2 Ground -> 5 Ground
3 RXD -> 2 RXD
4 TXD -> 3 TXD
Also, you need to set the communication protocol in the GS2, Mach3, and the COM port under windows device settings to be the same and they must be RTU. So for the GS2 you have only 3 options: 8-N-2, 8-E-1, or 8-O-1. I set mine up for 8-O-1.
Make sure to set you slave address in the GS2 and Modbus Serial setup to the same number. Set your base RPM and max RPM in the GS2 as well.
The attached images have all of my settings. My spindle now runs M3 FWD, M4 Reverse, M5 stop, and speed control 80-6000 RPM with the variable drive pulley set at 3000 on the Bridgeport. Also, don't forget to set the pulley 1 max and min speed under "Config->Spindle Pulleys.."
Copy and paste the scripts from this post
http://www.cnczone.com/forums/mach-mill/27820-modbus-run-gs-2-drive-2.html into your mach3 macros folder.
I spent more time than I should have figuring this all out. It only takes 5 minutes once you know what you are doing.