To select a note for the Note field within the trip, select the lookup button  next to the Note field: 

The note table will open:

Double-click on the appropriate note. The note that you selected will now display in the Note:

Other locations for note fields include: Passenger record on the ‘Report’ tab, a passport record in a passenger or personnel record, a trip leg on the ‘Leg’ tab, an aircraft note on the schedule board, an activity for a personnel record, a log entry on the ‘AC Log’ tab of an aircraft record, a log entry on the ‘Logbook’ tab of a personnel record, document records, training items and a quote on the ‘Quote’ tab.