Do you have a software firewall setup on your computer acting as your subsonic server? That could be why you can't see it from another computer.
What OS are you running Subsonic on? Check the read permissions on the file/directory to make sure it can be read.
As to your accessing from the outside world, get it working within your network first. The port forwarding only has to do with connections coming in form the outside world. Like tacgnol posted in his tutorial, you can sign up for one of the dynamic IP forwarding companies so you don't have to remember your IP. If you look in your router settings, you might see that it has a setting for DDNS (mine does Linksys WRT54GL) which will update the service with your IP so you don't have to run a client on your computer to keep it up to date. I use Dyndns.
Do you know what port you have Subsonic running on? I think default is 8080. You said in your first post you just use
http://localhost. That tells me its running on port 80 as it is the default port for a web browser to go to if you are not specifying it.