The icon in the Preview window just says 'E' because 'E'
should work.
What we discovered is that if there is no path to an application for 'Assign Default Application', then the 'E' key will not launch your editor if your launching preferences are set to "Let Finder open files". So, to get the 'E' key working, go to your Launching preferences, select "Assign default application" under "Default application to edit photos", and then Choose an application on your system. Once an application is chosen, you can then go back to 'Let the Finder open files", and the 'E' key should work.
In this case, it doesn't necessarily matter which application you choose as the default application, because this choice ends up being ignored and Photo Mechanic just goes with whatever Finder has set for the type of file you are working with.
For years, keying E in the preview window didn't do anything,
If Photo Mechanic behaves in a way that is not consistent with the interface, please let us know so that we can either fix the behavior or fix the interface. We will put a fix in the next build which will render the workaround above unnecessary, and the 'E' key will work even if no path is provided to an application.