I recently asked a plumber to install a cut off to water going upstairs... to save money by not heating the unused space and to reduce the chance of frozen pipes. His cut off only worked for a bar sink directly above the cut off (he put it on top of the hot water heater). The bathroom at the opposite corner is still flowing. How can I help him find where it comes in... shouldn't he know how?? Is there a better plan for preventing frozen pipes?

Yes he should have know because he should have checked his work before accepting payment and leaving. POipes in older homes sometimes are like a plate of spagetti going every which way. Your up stairs may have been an addition after the house was built.

Are you sure you hired a real air breathing plumber?

I do not believe that you can simply install a cut-off to protect the water from freezing. In fact, that's probably the worst thing you can do.

If you do that and don't drain both hot and cold legs of all water the water will sit in pipe and as it freezes it will burst the pipe. You of course won't notice this until you turn the water back on and then discover your upstairs pool.

You want the upstairs water supplies "Winterized to protect them from water damage." A plumber should know what he needs to do, without your help!