When adding a trip, only do so for one-way itineraries. For example, if you are flying from San Francisco to Chicago to Washington DC and back, only enter SFO–ORD–IAD, and not SFO–ORD–IAD–ORD–SFO. You will need to enter the return segment as a separate trip.
Tap on "Create Trip" from the Home Screen. You will then be prompted to select an airline. After selecting your airline, enter an origin airport (your departure point) by typing in the 3-letter airport code or the name of the city. Add any connecting airports followed by the destination.
After completing your itinerary, you can then choose the class of service (e.g. Economy, Business, or First) for each leg of the trip. Currently, the class of service defaults to "Economy". Thus, if you are flying in another class of service, tap on the "Economy" button to change your selection. Once complete, tap on any of the airports in your itinerary to view available lounges.