This is a configuration you will need to enable by going to Events then Manage. On the right side there is an ellipsis that has Settings. Turn on the Track Signup Mileage option.
If this is enabled, when you create an event under the roles, there will now be a few options to choose on how to track mileage. Duplie can either ask the volunteer after their sign-up, or an admin enters the mileage for the sign up. For the Pro plan Duplie will automatically calculate the mileage based on the user's profile address to the event location.