Having trouble fitting a 70-300mm Tamron Lens onto a Nikon D5000?

I got this lens in a bundle with the camera, it doesn't seem to want to fit on. Can someone please explain to me in detail (a dummys explanation lol) how to do it because the instructions don't make sense to me at all. Any help would be greatly appreciated.

It should just twist on in a counter clockwise manner. You should hear a light click.

I take it that you got the kit lens off your D5000.

Take the camera in your left hand, look from above. There's a white dot near the lens release button. There will be a similar mark on the lens (on your kit lens it's the same white dot that marks the current focal length setting). You can attach the lens if the dot on the camera and the marking on the lens are aligned. Then just turn the lens so that its marking is at the top where you expect the focal length indicator to be (seen from behind the camera it would be a clockwise rotation, from in front of it it would be counterclockwise).

Sounds like they gave you the wrong mount lens. Tamron makes lenses for a number of different camera systems, and it could be the retailer has picked up the wrong lens. Check on the packaging to make sure it is a "Nikon Fit". If it says Sony or Canon or Pentax then you've been given the wrong lens.

Do you need an adapter?