PuTTY to Ubuntu Karmic problem

It's not super easy
The problem is that when you are connecting to the outside, you are connecting to your routeur. So you must tell it to forward your request to the laptop.

If you know how to modify your routeur's settings, look for "port forwarding".
SSH is port 22.
Add a port forwarding on this port to the IP address of the laptop.

Also, the IP address of the laptop should always be the same to make this work. The best way to achieve this is also to set it in the routeur, if your routeur allows it of course. Usually, you can define a rule to say "this MAC address should always have this IP address". The MAC address is the unique identifier of a network card. Your wireless card on your laptop has one. So if you can, put a rule to link this MAC address to the IP address of your laptop.

Another solution if your routeur cannot do that would be to enter the IP address manually on the laptop. However, if you use your laptop outside of y our home, y ou must not forget to unset this if you connect to another wireless network... Not really user friendly.

Once those two steps are done, you can reach your laptop from anywhere using PuTTY or any SSH client.
Connect to the IP address of your routeur (the public IP address), and it will forward your call to the laptop.

There are other solutions, but this one just requires a small amount of work at the beginning, then it's done !
I hope I'm clear, don't hesitate to ask me questions.