Facing the Hyatt's registration desk (on the 6th floor), turn left and head toward the reception area. To your left you should see a set of elevators displaying a sign that looks like this:
(Sorry this first part is a bit abstract. The hotel staff didn't want me to take pictures, didn't want to give me a map, and in fact didn't want to have anything to do with me. I must remember next time to show up with conspicuous fistfuls of cash).
Get in the elevator and go to the second floor, labeled "Complexe Desjardins." When you exit on the second floor, you will see a food court.
The first part of the way, admittedly the most confusing, consists of crossing the Complexe Desjardins (see below, you are initially at the red circle, top left). [rpg: and, just to your right is a pretty good coffee shop. It's the closest good one if you're in the Hyatt.]
Follow the pathway to your right. You will emerge in the main plaza. [rpg: The poutine and hot-dog place is to the left from here; good ice cream roughly straight ahead; and toward the right and a little ways is a very good grocery store.]
Cross the main plaza, keeping on the right. Keep an eye out for the "Hallmark" store, and walk toward it, entering that branch of the plaza.
You will quickly arrive at a combination of stairs and escalators. Go down and keep going straight.
You will go trough a tunnel (that goes under René-Lévesque Boulevard). At the end of the tunnel, you will see a sign indicating "Complexe Guy-Favreau," which you are now entering. Turn right (or smash into the wall).
Go up the stairs / escalator.
At the top of the stairs, turn left. You will see a brown tunnel. Keep to the right, and go straight, along the right wall, toward the green signs. You are now crossing the "Complexe Guy-Favreau".
At the end of the hall, the hallway turns left, and you emerge in a white area. Turn right and then go straight.
Following the signs indicating "Palais des Congrès/Metro."
Once you reach the doors, turn left...
...and go down. [rpg: if it's not awful weather, you can go straight out these doors, and the Palais is right in front of you across a square and up some stairs.]
At the bottom of the stairs, you will have to turn right....
...and go through a long brown tunnel. You are now crossing Rue de la Gauchetière Ouest.
At the end of the tunnel you will see a moonlike sign pointing to the Palais des Congrès.
The sign seems to be pointing to a door, but do not exit. Instead, take the stairs up.
Congratulations, you are now in the Palais des Congrès. To reach the OOPSLA site, turn right shortly after going up the stairs, when you see the sign below. Go through the revolving doors.
You should see the large escalators leading to the OOPSLA site.
If at this point this is not what you see, good luck to you. With enough cash, you can survive for years in the underground system.