If you're adding a mileage expense, you'll have to add a description just as you would for a regular expense (i.e. "Visiting Client XYZ"). The amount however will be calculated automatically based on configured mileage compensation settings and entered distance. You may enter the distance travelled manually or using the map by entering source- and destination address. If you want to add an extra address to your route (if for example you've travelled via a different location to your meeting), press "add". The address will be added to your route directly. You may also want to check the "round trip" checkbox to have the distance counted twice before calculating the total amount.