Port forwarding on Mac OSX Leopard

Ok this might be a n00b question (owing to the fact that I am a n00b here, which is my excuse if this is the wrong section... sorry)... or maybe not, but so far i haven't been able to figure this out.

I just got a new iMac and I'm using the Airport to share the internet connection with my MacBook. I'm behind a D-Link DSL 502T router and I'm trying to forward some ports so I can play games online.

So basically my setup is like this:

D-Link DSL 502T router --> iMac (with internet connection sharing enabled) --> MacBook

Of course, I can get the ports to forward just fine to the iMac, but I've had no luck so far forwarding the ports to the MacBook. I've tried simple port forwarding, dummy programs, static routing and even port triggering to no avail.

If anyone has any surefire way of getting around this problem I'd be glad if you could enlighten this Mac n00b.

In theory it should be a simple job of logging into the router and setting up port forwarding in the router's own settings.

You should be able to set it up for the whole network, or each individual IP address.

theoretically yes, but because the laptop is behind the imac, its is as though it is behind a second router, and as such is on a second network of which the router with direct connection to the internet does not know about. and as such, it is impossible for me to forward the ports directly from the router without forwarding the ports from the imac.

so i'm gonna try to clarify my question (and possibly simplify it). How can I open ports on a machine running Mac OSX Leopard.

I'm gonna continue playing around with it, and if I get it figured out, I'll post a solution for anyone else with this problem.

Does it not work if you turn off the firewall on the Mac (since it would be enough to use the firewall in the router)?

i tried that, but the problem is it only allows incoming connections, and wont let me send any data to the server... im either gonna have to keep working on this one and start digging through the code or just give up... apple and their security...