Here's the user guide for your router
http://www.speedtouchdsl.com/documentation/ST780WL/R610/UserGuide/HTML/SIP/wwhelp/wwhimpl/js/html/wwhelp.htm?single=false&context=ug&topic=init

Just from having a quick look, it seems very similar to this one listed on the port forward site
http://www.portforward.com/english/routers/port_forwarding/Thomson-Alcatel/SpeedTouch536/Limewire.htm

Have a look through & compare them...if things look the same, you could follow the instructions for the SpeedTouch536. Don't forget to set up a static IP address first. You'll find a link for that within the portforward instructions.