Mac Users:
Under Mac OS Sierra and later, gatekeeper does not allow you to open the app.
This comes due to the fact that I am not subscribed to the mac developer program, which would cost me 100 bucks a year.

There are at least 3 solutions:

A) Send me enough money, and keep on sending money for the upcoming years, and I will subscribe to the developer program.

B) Tell gatekeeper to accept the app as secure:
Bring up the terminal and login with an admin account.
Navigate to the JoePass! folder.
Run these 2 commands:
sudo xattr -r -d com.apple.quarantine ./JoePass.app
sudo xattr -r -d com.apple.quarantine ./sys/JoePass-helper.app
Here`s where I found this information:
http://apple.stackexchange.com/questions/243687/allow-applications-downloaded-from-anywhere-in-macos-sierra

C) Download with another system:
In case you don`t know the admin password but have access to a system that does not set the xattr flags (this can be an older mac version, a windows pc, I didn`t check linux but it should work, or your smartphone, just to name a few), download JoePass! with this system and transfer it to your mac running Sierra. That`s all.
See https://forums.developer.apple.com/thread/48290

Have fun
Wolfgang