The compression tester does not tell you anything but a compression value. If the compression is down, the compression is down. And you most likely know that without the aid of the tester anyway, as many tell tale signs will allude you to a compression problem. Once you have tested the compression and established it is down, the only way to know why is to start stripping the engine
A leakdown tester will help you with what the possible compression issue is being caused by. This can be a great help in determing a cause before you start pulling anything apart. It maybe a simple issue of Valve adjustment through to just top end work or you may have a major seriouse issue below the head
As for a fitting, I would suggest yu find a diesel
guy and get him to send you an old worn out injector you can adapt to that you can keep in the tool box with your gauge.