I have a setup on my garage door that is similar to what is needed for MyGarage, though I am not using DS10As with OnAlert. As a matter of fact I don't have any security type modules that require OnAlert, so some of my comments on this may be a bit off.
The first suggestion I would have is to check your DS10A sensor in AHP for correct operation. When the door is open, does AHP indicate the correct state of the door? Likewise, when the door is closed? If that is not showing correctly, the problem may lie in the placement of your magnetic sensor. If the sensor is set up to be near the magnet when the door is open, remove the magnet and check AHP and see if it indicates that the door is closed. If that indicates correctly, put the magnet in place and see if AHP shows the door is in the opposite state. If not, it could be that you need to get your magnet closer to the sensor. Once you have things figured out in OnAlert, then jump into the MyGarage app and see if things are working.