Back to my mac

Okay so i have my .mac set up and im tryingto access the computer at my friends house but its not working. (vIA SCREEN SHARE)

EVERYTHIGN IS SETUP ON MY MAC correctly and his any ideas?

You have to have his IP address, I know that much...

My real question is how does this whole thing work if one or more of the computers in question is behind a router, does not have a public IP address and both machines are not on the same local network.

explain more please

Are you just trying to access his machine or do screen sharing or both?

just use his screen on my computer remotely

When you enable screen sharing, there is an IP address shown in the Window after you check the box. He needs to enter your IP address string as shown in the Window. It should be something like vnc://xx.xx.xxx.xx/

My problem is that mine is a 10.0.x.x internal addressing number and not a real IP address.