Mac OS X (Sierra or later) Workaround version of JES
I've discovered that if (a) you're a Mac user running High Sierra and (b) you have more than one version of Java installed, JES can no longer find the 1.6 version of the JRE that it needs to execute.
Grab this specialized version of JES 5.02, which is a workaround version of JES. This version requires JRE 1.6 available here: https://support.apple.com/kb/dl1572?locale=en_US. This JES5.mac02 version is literally hard-coded to run JES under JRE 1.6. (Upgrading to the latest version of Java will take more effort, and I'm planning on that for Summer break.)
Unzip the archive.
Drag the unzipped folder to your Applications folder.
Right-click on the JES icon, and choose "open."
Ben Shapiro has built a terrific version of JES that uses the latest version of Jython and is retina display aware. Grab it from here: http://web.eecs.umich.edu/~mjguz/uploads/jes-5-benhack-retina.zip