macOS High Sierra 10.13 introduces a new security feature that requires user approval before installing many downloaded applications that are not known to Apple. This approval is a one time process to "approve" FocusTune for use on your mac computer.
1.1. Set Security Policy
When you run FocusTune you may see a dialog that looks something like this:
Please make the following changes to your Security & Privacy settings to approve FocusTune for use on your system.
Open System Preferences, click on Security & Privacy.
1) You may need to click the Lock symbol in the bottom right corner of your Security & Privacy menu to make the other changes.
2.) Make sure that App Store and Identified developers is selected.
3.) Click: Allow, which will approve FocusTune to run on your system.
2.1. Apple GateKeeper Dialog Box
FocusTune is fully compatible with macOS Sierra and High Sierra, however, changes to the Apple Gatekeeper may force an extra dialog box to be clicked each time when launching FocusTune.
2.2. Copy FocusTune.app to Applications folder
After downloading FocusTune, the zip file should be uncompressed (double-click). Then FocusTune.app should be copied to the Applications folder. The downloaded version and the zip file can then be moved to the Trash.
2.3. Video Instructions
This video explains how to download and run FocusTune.app under macOS Sierra.