OK, in the model memory you have assigned for the sim, go to the servo assignment menu and re-assign channel 5 to an AUX channel that is controlled by a 3 position switch. Note depending on the template you are using you may need to go into the controls assignment menu and assign a switch to the AUX control you have selected in the servo assign menu.
Make sure in the differential mixer menu that differential is off, otherwise channel 1 will move more in one direction than the other.
How does the USB RC sim connector work, does this use patch leads from each channel or the SXRL link?