I thought the installation procedure for Electrum on OSX was to first extract the zip, then copy the resulting folder to Electrum's plugins folder (/Applications/Electrum.app/Contents/MacOS/electrum/plugins according to the Electrum console output of electrum.plugins.__path__) along with the rest of the plugins' respective folders... but when I restart Electrum & open Tools>Plugins, despite being in that top-level plugins folder, yours doesn't appear in the list & there's no add option.
(v4.5.8 & I cannot safely upgrade to a higher version of Electrum without buying a new computer)
Electrum v4.6.0 added better support for third party plugins:
* Third-party plugins:
- Electrum supports the installation of plugins distributed by
third-parties as ZIP files. While it has long been possible to
install third-party plugins when running Electrum from python
sources, the same is now possible when using desktop binaries
(Windows, MacOS, Linux). Third-party plugins are installed as ZIP
files in the user's electrum data directory.
- In order to prevent plugin installation by malware, third-party
plugins can only be enabled if the user enters a plugin
authorization password (distinct from the wallet password).
Setting up that plugin authorization password requires
administrator permissions on the local machine; a
password-derived public key must be written in the system.
Plugins probably works a lot differently before 4.6.0, no idea if it's possible to do the same or how to do it. 