Using older lenses on the K100d Super

We were at Jessops today looking for a camera for my wife; we looked at the K100D Super, and I tried my SMC-Pentax-A F1.4 50mm lens on the camera. I put it at the A setting, but the camera refused to take a picture (pressed shutter button, nothing happens).

It was all a bit rushed as we were also looking at other cameras - but what should I have done to allow the K100D to use my older lens?

Thanks,
--
Tim