M42 adaptor Minolta 7D does not see lens attached via M42 adaptor.

Quite right too. When the M42 adaptor is fitted there is absolutely no exchange of information between the lens and the body: M42 lenses pre-date all of those later developments. Your camera will also be unable to stop down the lens automatically when taking the picture, most M42 lenses don't even stop down automatically when connected to an M42 body.

You need to do things the old-fashioned way. Your camera needs to be set to meter manually, shutter priority mode may also be used. In manual mode you focus the lens as normal with the aperture ring set to the lowest aperture number (i.e.aperture is wide open).
You then make sure that the lens in in manual mode as well and stop down to whatever you want, if the image remains bright enough then you can adjust the precise focus using the hyperfocal principle if you like which takes advantage of the increased depth of field of a stopped down lens.
In manual mode, you then tell the camera what aperture you have set (read it from the lens barrel) and set the shutter speed using the camera's light meter to guide you. If using shutter priority mode then the camera will choose the shutter speed for you.
Check everything is set as you intend and press the shutter.

It all sounds long winded but is exactly how many of the world's greatest photos were taken and soon becomes second nature. You also learn far more about the relationship between aperture, shutter speed and ISO settings and will be able to talk about reciprocity like you know about it!