How do I use the GNS 620 Trainer provided by Garmin?
I found all the information needed in the "Trainer's User Guide.pdf" that was installed with the software.  

In short, launch ONLY the GTN - G600 Trainer.  Do not have any other components started.

3 windows should launch: GTN Trainer, G600 Trainer and Navigation Control.

Give it a good 30 seconds to load up, then on the GTN Trainer window, click "Power On".
All windows should come up and be active.