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One thing that might be the problem is your aperture ring setting. There is a menu setting that allows or prohibits the use of a lens with an aperture ring.

If you click the menu button, then click the left arrow, you will be in the custom setting menu screen. Next hit the up arrow twice. You should be over the "Using aperture ring" choice. Click the right arrow. Set this setting to #2 (permitted) and click the OK button twice.

Shut off the camera, mount one of the older lenses, and turn the camera back on. You should get a prompt on the LCD display for the focal length of the lens if it's a prime lens (not sure what happens with a manual zoom lens). Select the focal length.

Couple other things to remember....obviously auto focus will not work so turn the button on the left side of the camera near where the lens mounts to MF. And finally, depending on the lens, you may need to be in manual mode, or limited to manual and shutter priority mode. The old M-style lenses will most likely be manual only. I believe the A-styles you have some auto features but can't remember which ones.

Hope that helps get you going in the right direction!

I have several manual lenses for my K20D. I have a really old one that needs an adapter ring to mount, but works absolutely splendid! It really is nice to be able to look on Ebay or at the local camera store's used gear and pick up and excellent prime lens for $50! And Pentax made some of the best quality primes ever.