When I try to open the app file I receive a message stating that the developer is unknown and therefore the app will not be open. My friend has Sweet midi player on his mac so that is obviously not the incompatibility. I am stumped.
Mountain Lion was a long time ago, so I don't remember exactly how it worked back then, but at least in modern versions of macOS, if you right-click (2-finger click, ctrl-click) on an app icon in the Finder, and select "Open" from there, then you will get a slightly different dialog that lets you override the code signing check and open it anyway.
Or, after being blocked when opening an unrecognized app, if you go to
System Preferences > Security & Privacy > General > Allow apps downloaded from:, it will have some special temporary UI telling you that an app launch was blocked, and asking if you want to let that app launch.
In that same place in System Preferences, you can change your global settings for what kinds of apps can or can't be launched, but I don't recommend setting it to anything less than "App Store and identified developers".