The Portland airport is served by the TriMet Max Red Line light rail. Board the Max Red Line toward City Center & Beaverton and ride 10 stops (about 26 minutes) before getting off at the Convention Center MAX Station. All Max trains run at least every 15 minutes, most of the day, everyday. 
 
(See TriMet's frequent service map for rail and bus here).

Thanks to TriMet, free 7-day Hop Fastpass Transit Fare Cards were mailed to those who requested them by April 12. If you did not request or receive one, TriMet Hop Fastpasses will be available at registration.