How can i setup Port Forwarding behind a wireless gateway?

My configuration is as follow:

I got a regular modem-router (non-wireless, 1 ethernet port, provided by the internet provider) which is connected to a wireless gateway through ethernet (the model of that wireless gateway is Asus WL-330g). My regular modem-router connects to the internet and the wireless gateway acts as a DHCP server to the machines of my private network (1 PC, 1 Laptop, 1 PS3).

I want to know how can i setup port forwarding for the machines of my network.

Whatever i have tried my Vuze Client still complaining about NAT problems , so do my PS3 (NAT type 3) which seems to reduce my Playstation's connectivity to the internet (problems that has to do with voice chat to some games).

I have tried to port forward certain ports to both the regular modem-router and my wireless gateway but no luck.

I would gratefully receive your help.