I have a 1971 seeburg firestar jukebox - the zero (0) does not seem to work when you punch in the 3 numbers for the selection - so none of my selections that have zero in it do not play - also I do not have a key that works the lock on the back at the top -

Same key that operates the locks on the sides unlocks the top, but top might be sticky. Push down on it.
Order keys if you don't have them. It's key F291. GOOD locksmith can make it or order one off eBay.
Your jukebox is a USC2. Use that model number as a reference.
Keypad "0" contacts are probably dirty or broken, or the connectors to the keyboard or black and gray boxes might be loose or need cleaning. Circuit board might be dirty (YES! This is a common Seeburg issue). To clean it of check for broken contacts on the "0" key, you'll have to remove it and open it up. Putting it back together can be tricky. Be patient.
Worst case, you can install a replacement keyboard.

If you have someone come out, be SURE to have them give the jukebox a good lubrication. Seeburgs are complicated machines and need periodic lubrication.

Also, buy a manual. Jukebox repairmen don't usually have the manuals for your jukebox. Too many.