From developer.apple.com:
To develop without a certificate, each time you launch Safari, you need to tell it to load unsigned extensions using the Develop menu:
Open Safari and choose Safari > Preferences.
Select the Advanced tab, then select the “Show Develop menu in menu bar” checkbox.
Choose Develop > Allow Unsigned Extensions, enter your password, and click OK. The Allow Unsigned Extensions setting resets when a user quits Safari, so you need to set it again the next time you launch Safari.
Choose Safari > Preferences and click the Extensions tab. This tab shows the localized description, display name, and version number for the selected Safari app extension. It also provides a more nuanced message about the permissions for the extension.
Find your new extension in the list on the left, and enable it by selecting its checkbox.
Close Safari Preferences.