But if both computers are in different networks, things can get complicated ... SSH tunneling to the rescue. In a Terminal in your local computer type:
ssh -L 5900:localhost:1202 [username@]your.remote.computer
Then launch ScreenShare and when asked for a Host type:
That should do the trick!
(btw, the ScreenShare app is located in
/System/Library/CoreServices/, at least in OS X 10.6)