Hi Davide,
I was also on OSX 10.9 and EM 7.2.1.6 when tested your extension, and there was no installation issue. Could you try to install the same extension on another machine?
If it is not help, would you please collect log to us?
1. Launch Extension Manager, and checked the check box "Enable detailed logging(restart required)" under "Tools" -> "Preference".
2. Restart Extension Manager and reproduce your issue
3. Send us ExManCoreLibrary.log file in /Users/<name>/Library/Application Support/Adobe/Extension Manager CC/Log/ to exman.beijing@gmail.com
Thanks,
Huiyong