Sorry! Of course you cant add an else to that condition. What you should do is create an event with sub-events. Something like this:
Condition - Button pressed
sub-event - flying skill >= 5 - Do this
sub-event - flying skill < 5 - Do that (or you can make THIS an 'else')
All this will be in one event.
To add a sub-event just mark the event and press S. Or right click on the event and choose sub-event.