There's a map here:
Fourth from the bottom is a "Vancouver to the Expo MAX station" link to a pdf map. It's a little weird until you've gone through there, but once you've done it it's no big deal. The main idea is to track down the path maze around where I-5 touches down by Delta Park, then once you've figured out the way to Jantzen Beach, find your way to the no-mans-land in the middle where the freeway crosses.
Depending on when you're headed north, there should be cyclists headed through often enough. You might be able to follow someone if you get lost or otherwise bewildered.