your wan address can be gotten from your router. You need to forward port 80 on your router to the local IP address of the linkstation (usually 192.168.x.x)
Once this is done and your away from home, open a browser and in the address bar type : xxx.xxx.xxx.xxx:80 (xxx.xxx.xxx.xxx denotes your wan IP) => enter => you will then be asked for the username and password for the linkstation.
You need to bare in mind that your WAN IP address can change, so a better solution would be to use dyndns (if your router supports it, but most of them these days do :))