I see leaves rustling all around me (I'm at 14th Street/Union Square)!! Do I have to do some thing to make the pokemon spawn, or is it just random? I've tried walking above them with no success.
It is not guaranteed that a Pokemon will spawn in rustling grass, but there is an improved chance of it doing so.
Other ways to improve your chances are using Incense, or setting up Lure modules at Pokestops. You can also tap on the box in the bottom right corner to bring up a radar showing how close various Pokemon are. The fewer footprints, the closer - and if there are no footprints, tap on the Pokemon to show its location on the map!
When a Pokemon spawns, it may be very small on the map and hard to notice, but your phone will vibrate to alert you.
As far as I can tell, Pokemon are no more likely to spawn in parks (such as Union Square) than they are on a regular street.