If installing or running a Safari extension isn't working, it is likely because the extension is unsigned. To enable unsigned extensions, you will need to enable the Safari developer menu first.
Enable Safari developer menu
1. Pull down the “Safari” menu and choose “Preferences”
2. Click on the "Advanced" tab
3. Check the checkbox "Show Develop menu in menu bar.” This enables Safari developer menu
Enable unsigned extensions
1. Go to "Developer" menu in Safari
2. Select "allow unsigned extensions" (note: earlier version of Safari may not have this option)
3. Uncheck "disable extensions" in the same "developer" menu to allow extensions
4. Install and run your extension again.