If you are using an older computer, especially Mac 2009, Open GL is not supported.
Unfortunately that machine is no longer supported by Apple, and as a result they no longer keep new apps as supported on that hardware, and it will not be able to run MainStage as a result of the physical hardware being unable to process the data needed to run MainStage.
There are certain driver updates for that line of graphics cards on the Windows platform that would allow it to process newer versions of OpenGL, but unfortunately Apple has chosen not to update drivers or release updates for that card.