You should use the zip file from the asserts of the latest release and install it with blender.
Releases:
https://github.com/tobi-be/BlenderEdmExporter/releases
Be aware that this exporter is not thought to export downloaded models and have a working model with 2 or 3 clicks.
Install:
Download io_BlenderEdmExporter.zip from asserts of the latest release.
Blender: Go to Edit->Preferences->Add-Ons, press install and choose downloaded zip file
Activate the add-on
It is written in the readme. Please read it.