If there is a GPS flight mode configured into the flight mode switch, Revo will not arm unless:
- Configuration -> Attitude -> Settings -> "Attitude Estimation Algorithm" is set to "GPS Navigation (INS13)"
- HomeLocation is set
- Mag (magnetometer) is calibrated
- Mag is green color in system health
- GPS is green color in system health
Besides that, all the high current motor wire pairs (and triples) must be twisted together with the pair becoming one cable. If not, you may get a "fly away".
I recommend that you play with "Velocity Roam" flight mode because it is simple, safe, and more interesting than "Position Hold". If you leave sticks in the middle it is the same as Position Hold.