I login to the WRT54G using IP address 192.168.1.1 from the LAN side. By default I believe the username and password are both "admin". This may or may not be the same for your Router.
For greater security, I do not use the default password. I have reset it to use a password with greater security. Check your user documentation for the specific login information that came with your router.
Basically, what you want to do here is decide what application you wish to trigger ports open with. Assign the Application box the name of the application you will be triggering with.
If your application only requires a single port, enter that port as the Start Port and End Port in the "Triggered Range" section.
If the Triggered Range is also the Forwarded Range, enter that same port range in the Start Port and End Port under the "Forwarded Range" section. And, be sure to "Enable" the triggering by checking the Enable box.
For example, if you use Google Talk, you will only need to open port 5222 .
I give the Application section name "GTalk". Then, I assign the Trigger Range as Start Port = 5222 and End Port = 5222. Then, I set the triggered Forwarded Range Start Port = 5222 and End Port = 5222 and then check Enable.
Skype picks a random port to use. So if you are a Skype user, you will need to navigate to Skypes Tools Options Advanced Connection window to find out what random port it has selected for you. I myself pick my own personally specified port, which Skype allows you to do. Whatever port is selected, you will need to configure your NAT-Router to "trigger" on this port.
I use two separate settings for standard SIP based VoIP.
One for triggering SIP ports 5060-5065, and another trigger for the SIP RTP ports 16384 - 16482. In the first case, I use SIP triggers to open SIP ports. In the second case, I use the SIP triggers to open the RTP ports.