I just changed the air pressure switch on my SpaBuilders controller for my spa. I noticed that the bare ground wire attached to the left side of the controller was not attached at the other end. I searched, but could not find a likely place to connect this.

Where shall I connect this ground wire on my spa controller?

The bare ground should also connect to the motor, heater and back to the bare from your feed line. Look for small screw connectors that screw into an aluminium looking block. There should be a similar attaching point at all of the names points. Your breaker should also be a GFI in the panel or sub panel.