It's much further west then you'd probably expect. Can't say for certain but I believe it's almost at the very North-west of the lower island, near the seafront, beside where the big mountain area is. It's easy to spot once you're in the right area because you'll see the large screen.
Seriously, if you look at your map, there is a circle that might kinda, just maybe look like it would be a drive in theatre. Took about five minutes to find it. And maybe that might not be it but it's super close to that area.
Now Playing: Saint's Row 2, Fallout 3. Waiting For: Just Cause 2, and Prototype.