After you install Locket, it can occasionally take up to an hour for the widget to appear in the iOS Widget Gallery. If you don't want to wait, restarting your iPhone will make the widget appear immediately.
Steps to add the Locket widget
Go to your Home Screen
Hold down on any app to edit your Home Screen
Tap the Plus button in the top left corner
Search for Locket and add the widget
If you still don't see Locket, try restarting your phone or waiting up to an hour.