I'm finishing up an F550 with a DX8 transmitter.  I've set it up according to a video I saw on youtube.  When I initiate the sequence to activate the motors, all 6 motors immediately begin spinning at an "idle" speed.  They will not shut off using the throttle.  I've tried moving the end point of the throttle but to no avail.  Any ideas???

This is normal for the motors to spin at idle.  This lets you know the copter is armed and ready to go.  You can do a CSC to disarm and they will stop.  If you go to above 50% throttle then you are telling the copter to begin raising in altitude.  Put you Endpoint back to default and I would do another transmitter calibration in the Naza Assistant to be safe.