You need to get up on the ledge (I think you're referring to Philly). The ledge is in front of you when you start that level (down the stairs, and up). There are two ways. Turn around, and gain as much speed as possible, and jump off the ramp. That rarely works and is a waste of time. The other way is to go where there is a long set of gaps (with benches in between) and on the other side is just a long grind (with grass). In that grassy area, you'll find a wire you need to grind (which on it has the video tape). Takes a lot of balance, and speed, so use that long grind to your advantage.
The venice ledge is in the back of the level. Basically, you need to find the sidewalk which on the right of it has sand (if you fall in you go back to the starting point). Around there there's downward sloped ledge which is the venice ledge. You need to do a special grind, I think it's a tail slide or something so it might take a while. I think you can find it by just going around the map hugging the right wall (grinding the fences may help).
I'm doing this from memory (and haven't played the game in a very very long time), but I'm almost certain that's how you get to both.