The more detailed map on the museum website is pretty good. I've never actually been in the museum but I used to play paintball fairly regularly at a field in the hangar behind it (which shares the hanger with an indoor soccer field, or at least used to).
It has been a couple years since I was last out that way, but my suggestion for anyone driving would be to take Sheppard to John Drury Dr. and enter Downsview there, turn left on Carl Hall Rd. from John Drury, and then the museum is a ways down Carl Hall Rd. on the right side immediately after a set of railway tracks. The museum is (or at least was) quite well marked, the soccer and paintball fields were harder to spot because they are in the next row of hangars back from the road.
The A on this map is a little bit further down the road than the museum and pointing at the correct building:
toronto aerospace museum, toronto, on - Google Maps
I occasionally met people who had walked from the subway to the paintball field and got the impression that it is a fairly long walk (the airfield is very large) but I've never taken that route myself.
If somebody knows a good spot to meet near the Downsview subway exit a while before the museum opens (11:30 maybe?) then I would be fine with giving few people a ride from there to the museum. If we can't come up with a location to meet there and some Pentax signs to find each other then meeting at the museum would be easier.
How many people are planning to take transit?