In session [Names] where you are using (outright) event pair numbers, enter them using [Event numbers] [F7]. As you enter the event numbers the names in the event list are shown in the session names.
Another way is to use the [Event Pairs] option [F3]. This gives you a list of the pairs in the event, then click the pair you want, and click select. (or simply double-click the pair) This places this pairs number and players into the current slot.
When this event pairs list is visible [F3] the entry of event pair numbers using [F7] behaves slightly differently from when not visible. The names are not transferred from the event to the session, just the number is changed.