Screen sharing between Macs is very easy as long as they are located in the same local network.
Once the are in different networks it gets a little more – no not complicated, it’s just not that obvious.

Looking for a solution I found a great article “Remote Control: Screen Sharing in Leopard” by Ryan Faas on Peachpit.
Here is the part (see article page 3) that helped me out:

“You can, however, connect to computers on other network segments if you know their IP addresses.
…
To do this you’ll need to launch the Screen Sharing application manually. The application is located inside the Core Services folder along with several other Mac OS X components that are normally launched as part of operating system (including the Finder).

This folder is located at the following path (beginning at the root level of your hard drive): /System/Library/Core Services.”