Gamespy arcade wont let me to host, it says my internal ip is not the same as my external, how do i fix this?

i need a detailed explanation to this, i am fairly new to routers but am not computer illiterate. I have a speedtouch 530 dsl modem, what exactly do i need to do to be able to host with gamespy arcade?

http://portforward.com/english/routers/port_forwarding/Thomson-Alcatel/SpeedTouch530/GameSpy_Arcade.htm