Can't use matchmaker on different computer?

I got my networking going in my game and wanted to try it over the internet. So I tried the Matchmaker and it seemed to work, I could join myself on the same computer, but then when I ran the game on a different computer it just said "Failed CreateMatch for appId=Invalid]-addres:,port:0"... and then everything set to a bunch of zeros.

Have I missed something or why would it only work on my machine?