Forgive me if you already know some of this. I'm just trying to be thorough...
I also have an Airport Extreme. I'm not at home right now and I can't remember exactly how to do it. Within the Airport Utility, you are looking for something called "Port Mapping", or "Port Forwarding" or something like that. What that does is allow you to specify a port on the internet side of your router and forward it to a specific IP address + port on your local network. If you're not already using static IP addresses, you'll want to assign one to your mac pro. Once you have that, you will want to forward it to <IP Address of your Mac Pro> at port 22 (port 22 is the standard SSH port).
Now the question is what port do you want to open on the internet side of your router? You could just open up port 22. Now I'm no security expert, and I might be fooling myself by even doing this, but I use a different port on the theory that using an obscure port number is more secure. If you use a different port, you'll need to specify it on the ssh command line when you are logging in from outside of your local network (-p option)
That will take care of the router part of the equation. If you are not familiar with setting up SSH, I recommend you do some googling about how to secure SSH on your computer. OSX runs OpenSSH, which is a freely available ssh implementation used on many unix operating systems. I recommend you google for securing openssh. You might want to, for example, set up an ssh key and enable passwordless logins. This means you cannot login by typing in your user's password, but instead need a specific key.
edit: you'll also need your internet IP address. I setup dynamic dns, through dyndns.org, as my internet provider gives me a dynamic ip address.