Double-click to unpack Microstockr-mac.zip, then drag & drop the app icon to your Applications folder.
Inside the Applications folder right click the Microstockr app, select Open and in the pop-up click Done.
Go to System Settings, Privacy & Security, select App Store & Known Developers then click Open Anyway below.
Click Open Anyway in the pop-up window to launch the app.
You're ready to go. Enjoy!