Download Fillable from the website.
Launch Fillable.
You will receive an alert saying "Fillable.app can't be opened because it is from an unidentified developer." Click OK to dismiss the alert.
Open System Preferences
Click on Security & Privacy
Select the General tab
Click on the Open Anyway button.
You will receive an alert saying "Fillable.app can't be opened because it is from an unidentified developer." Click the Open button.
Fillable will be in the dock
Click on it’s Dock icon.
Now you’re in business!