1.- Please check in which flight mode you are currently on: Stabilize, ACRO, AltHold or Loiter mode
*When trying to arm in Loiter, you need to wait until you have good GPS signal. The led will be green indicating it is ready to take off.
2.- If the led indicator is yellow, this indicates the vehicle is not ready to take off. If you are using Mission Planner or APM planner, it will show why you can't take off, these are called pre-arm safety check messages. Click here for information on how to clear them.
3.- Try to arm with the Throttle down and to the left, if this works, then you need to reverse the rudder of your RC.