#1. "RE: AF-S 55-300 lens on D3100 "lens not attached" message" In response to In response to 0
Welcome to Nikonians! The Nikkor 55-300mm f/4.5-5.6G ED AF-S VR DX is fully compatible with the D3100.
The dot on the top of the lens barrel at the back of the lens should be at the 12 o'clock position when the lens is mounted on the camera. It is only an alignment mark and has nothing to do with how the camera detects the lens.
Are you inadvertantly pressing the lens release button on the camera body?
Is the lens fully seated and locked in the mount? Try rotating the lens back and fourth in the mount to check. If the lens is fully seated and locked in the mount and the problem continues, remove the lens and clean the electrical contacts on both the camera body and the llens mount. Make sure all of the electrical contacts on the lens project out the same distance. They are spring loaded and can get jammed in. The remount the lens: align the dot on the lens with the dot on the camera body then rotate 1/4 turn counter clockwise until it locks in the mount. it should work. If not it is more than likely a problem with the lens. Good Luck and Enjoy your Nikons!