Macro tube extension is really dark? Nikon D3000?

I have a nikon D3000 dslr camera with a 18-55mm autofocus/manual focus lens and am doing coursework on macro photography, to save on a macro lens I bought a set of macro tube extenders, there's three different ones that fit toghether, they're very simple and just screw in together (http://www.amazon.co.uk/MACRO-EXTENSION-TUBES-CANON-Camera/dp/B001CWZE6A) thats the one I got.
I've attached it fine but when they're attached, either one, two or three throught the viewfinder is very very dark and when i take a photo its pitch black even ewith a shutter speed of about ten seconds its still a bit dark. Just wondering if anyone knows how to fix this? Thanks

Added (1). The link i sent was wrong! Http://www.amazon.co.uk/Macro-Extension-Nikon-Camera-D3000/dp/B004TW4YI4/ thats the right one sorry

You need tubes that are compatible with the controls of your camera. Return those and get a set for Nikon. How did you get the mount to work on your Nikon if the tubes are Canon?

I assume you got the nikon version of those rings.

Problem here is your lens. The 18-55 is a g-type lens that lacks aperture ring. Because of that with the rings the aperture is always fully closed. What you need to do is find a d-type lens or, if it's just for macro, any old Ai or Ai-s lens (nikon's 50mm f1.8 E-series is plentiful and cheap). On those you can set the aperture using the ring on the lens.

Even with one of those you'd need to set the camera to M mode otherwise it will not work.

Other alternative is to buy a set of rings with electronic coupling like this: http://www.amazon.co.uk/...B002YIY44O

Nothing to fix -- it's doing exactly what it's supposed to do.
Extending the lens out using extension tubes involves light loss -- welcome to macro.

Put more light on your subjects.

What lighting are you using?

i use a bellows, and have problems with lack of light…

use a spot light…