The best way to get between the Portland Airport and the Oregon Convention Center is to use MAX, Tri-Met's light-rail system. This train will pick you up right at the airport and drop you off right at the OCC's doorstep, all for $2.30. The OCC is less than half an hour from the airport by MAX.
You can also hop the MAX into downtown Portland free and take in the night life. Downtown is only 5 minutes away by MAX.