Did you press the location button to follow you? You have to do this on the iPhone 4, so hopefully that will work with the 3GS.
"Directions are displayed on the screen in the same style as "turn-by-turn" but users can swipe manually just like the iOS 5 version, or the directions change after you reach the turn."
This is not the case on my 3GS. I tried it on my way to work this morning. While the directions were displayed in "turn by turn style", they did not change as I reached/made turns - I had to swipe to get to the next step. Don't know if the 3GS isn't supposed to have this feature or not.