Keep at it! Try single point autofocus on the Nikon and the 55-300mm lens with the moon (this way you tell the camera exactly what to focus on). Start with your zoom lens set around 100mm with an aperture of f5.6, ISO1600 and 1/500th of a second. You should be able handhold this and get a good shot (i.e. you don't have to stay outside long!). Will be interested in how you get on - happy for you to post your image here and I will give feedback.