Ohayo izzzu! (Sorry, I don't speak japanese, I just knew a japanese once.)
Your Mac probably has
a firewall, you need to open the specific ports (usually 4662 TCP, 4665 UDP and 4672 UDP
dangit, i forgot the tcp+3 some posts earlier).
If you own a router you have to forward these ports to your IP.
First of all we need a static IP in your routers ip-range.
Access via system->system administration->network (or similar) the properties of your network adapter.

The blue underlined number should be in the range between 2 and 254 and only occur once in the network.
Don't forget to set the DNS in the same little program.

Now you're ready to access your router by typing the IP into your browser. (sorry cannot provide screenshots atm, 'cause I disabled wireless config.
It's different on the most routers, but there should be a menu item like 'port forwarding' or in the case of my WRT54G 'application and gaming', where you could forward the necessary ports (as mentioned above)
If you have a more common router, google will show you instructions for your model.