Why do my pictures come out dark?

I recently received a Nikon D5100 for my birthday and have enjoyed it very much with no problems at all, except for when I switch lenses. My dad has lenses from his old Nikon N80 which fit my camera. The only one that I have had a problem with is the macro lens. The pictures come out very dark. I have tried adjusting the apature and the exposure time. Any reason for why that is?

Dark = underexposure.

You need to use the light meter in the view camera to assure that your exposures are correct.

What macro lens? Is it complaint with your D5100?

Try this. Using one of your AF-S lenses, get a meter reading in the manual mode. Replace the lens with the macro and use the same settings. The settings will be the same.

1/250th second at f/11 is the same exposure no matter what lens or camera you are using.

Entry level Nikon's (D3000, D3100, D3200, D5000 and D5100) light meters will not work if you are using manual lenses. If the macro lens is a manual lens, then you need to use a different way to determine the proper exposure

Have you tried raising your ISO.

Just a guess, But, come out of Manual Mode and use Aperture Priority Mode, you set the aperture, half press the shutter and the camera will tell you what shutter speed it's having to use with the light it sees.

Probably a lot slower shutter speed than you were expecting as with Macro your camera is only seeing the light reflected off a small area, add to that the need to use a small aperture (large f number) to get any depth of field which makes the light the camera sees even dimmer and you can see why your images are dark.

Shutter times can need to be as slow as 1 second in dim light with a low ISO.

Manual Mode is only good when mixing ambient with flash and other specialist things, there's no advantage in using Manual Mode till you know what your doing and why, and even then Aperture Priority, EV compensation and using the built in Histogram will get you far closer to spot on metering than any other method. That's how digital cameras are designed to be used and why they always include those functions.

You need lots of light or flash to get good macro shots, or long shutter times and lots of noise.