I was just looking over the info on the PAX site and I noticed that the directions for all points north (southbound into Seattle) indicate that people should take the Stewart St. Exit and then travel around 15 city blocks on side streets.
For people unfamiliar with the city, and unaccustomed to city driving, that might be a bit intimidating. Although they only have to make like, 3 turns, its over the course of many city blocks.
Why not recommend Union St Exit? It's literally right under the convention center and better yet, only requires like 3 blocks of city driving. It couldn't be easier.
Exit on Union St. move to the right as you get on to Union.
Take the FIRST right available onto 7th.
Take the next FIRST right available onto Pike.
Take the next FIRST right available onto 8th.
Drive half a block up the hill on 8th, parking garage entrance is on your left.
I made a cruedly drawn map:

The only downside to Union St exit is that the "Downtown Exits" section of I-5 southbound can be chaotic and a little confusing, especially if you're new to the area. Another potential downside would be you're in the core of downtown and there will be lots of pedestrians crossing so making right turns might be a little hectic. But thats the way I go if I'm ever in the area.
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Oh well... now people know a shortcut they can try if they're feeling lucky