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"RE: AF-S 55-300 lens on D3100 "lens not attached" message"
As Marty said, the little white "dome" is just a physical indicator to help you align the lens when mounting. It has no other function, and isn't part of the problem here The "dome" is a recent innovation from Nikon - Nikkor lenses launched prior to the last year or so have instead a simple white painted dot to fulfil the same function.