If the machines are on separate networks then you need to connect to the machine serving the files by it's external IP address. With this you should be able to connect to it using Finder by clicking Connect to Server... from the Go menu. If it is a Mac serving the files use:
and it should prompt you for a username and password.
Alternatively if it's just file access you desire and are not set on using a GUI, I recommend SSH and SCP: two command line tools.
SSH (Secure Shell) allows you to login to a remote machine and execute commands as if you were using a shell on that machine.
SCP (Secure Copy) allows you to securely transfer files between *NIX based machines using SSH protocols.
For information on how to use these, open a terminal and type either
man ssh or