Dark Images with manual lenses.

There are a couple of things that need to be checked with a K-r to shoot with manual lenses. Firstly you need you need to set 'use aperture ring' to 'permitted' in order to use manual lenses.

Press the menu button.
Press the left button on the four-way controller once to get to the Custom Settings menu
Press the down button on the four-way controller and select Using Aperture Ring
Press the down button on the four-way controller, select Permitted and press the OK button.
Press the Menu button to exit the Custom Settings menu

You need to set the focal length of the lens when you turn the camera on, though this isn't vital if not accurate.

Are your old manual lenses M or A? Do they have aperture rings and, if so, do they have an 'A' on the ring?

With M lenses you should be able to simply set the aperture ring to the desired setting. Select manual (M) on the dial on the camera.

When you have composed the shot press the green button once. This will take a reading for you and set the shutter speed to match your chosen aperture. I tend to find that this underexposes a little so usually reduce the selected shutter speed by a couple of steps.

A lenses can be set to A on the aperture ring and then you should be able to use AV mode to select both aperture and shutter speed.

Hope this helps. If it doesn't come back and let us know what happens if you follow the above.