If you're trying to install the Sidebar on a Mac and you get the following warning...
"HealthEngine Appointment Sidebar.app" can't be opened because it is from an unidentified developer.
That simply means it's not registered and available on the Apple App Store.
To get past this warning, open your system preferences and go to Security > General (tab) > Allow applications downloaded from: Change it to 'Anywhere'.
If it's greyed out, click the lock on the bottom left of that dialog box, then enter your Mac password. You should be able to change it after that.
Once you've made that change, try the Sidebar installation again.