I have recently bought a macro extention tube for my nikon d5100 with 18-55mm lens?

I have recently bought a macro extention tube for my nikon d5100 with 18-55mm lens?

Added (1). but for some reason after adding the extension tube between the camera n lens. Its dark n bluss when i look through it and on the camera screen it says lens not fitted or attached. Any idea? Please advice. Cheers

Added (2). how do i make it work and how i set it up interms of getting rid of the drk blurr ness n even if i have to use it in manual mode but how exactly? Cheers

Did you have a question in there somewhere?

Unless the extension tubes have electronic contacts to communicate to the camera, then you will have to shoot in the totally manual mode. You will have to use the cameras lightmeter in the stop down mode or use an external lightmeter

Fitting or mount, may be a DD mount. Fhotoace is right

Here is a step by step explanation.
http://www.wikihow.com/Use-Extension-Tubes-to-Shoot-Macro

Or, if you prefer YouTube,