I have a D-Link DI-604. I've had it less than a month and I'm still trying to figure out this stuff, too. Here is what I've learned. Maybe it will help you if your D-Link 500 software is similar. I've at least downloaded music, but no one has uploaded yet.
After all this typing, I hope it helps, somewhat. I tried to get some help like this a few weeks ago from a Senior Member, but I guess they have too many emails to respond to.

In OSX, you have to do set BOTH the system firewall and the router firewall to be turned off in port 6346. System 9 doesn't have a built in firewall. A little bit of a pain, but not too bad once you've done it a couple times.

System OSX:
Go to System Preferences -> Network -> Show: Network Status. That should have the address number of your computer / three numbers, a period, three numbers, a period, one number, a period, three numbers, a period. Write that number down or keep the window open.
Then Go to System Preferences -> Sharing -> Firewall -> Scroll the blue bar down and check the box, Gnutella/Limewire (6346).
This part I'm not sure of and it doesn't seem to matter. Go to System Preferences -> Sharing -> Services -> click the Personal File Sharing box.
REMEMBER to turn these off when your done using LimeWire.

Start your browser and type in the address of the router in the search bar. You can do this stuff off-line. Click Advanced -> Firewall. Click the Enabled button. In Source -> Interface pull down the arrow and choose WAN. In Destination pull down the arrow and choose LAN. In the IP Range Start box put the address number of the router. In the IP Range End box put the address number of your computer. Protocol - TCP. Port Range 1 - 1, or whatever port your ethernet cable is plugged in. Under Schedule you can pick the time you want the port open.
THEN CLICK THE APPLY BUTTON. If you don't do that, it just erases everything you just typed in. The next time you want to open the port, click on the icon that looks like a piece of paper and a pencil and set the time you want to have the port open, like Saturday from 10:00 - 11:00 AM.
To download stuff, go to Advanced -> Virtual Server. Click the Enabled button. Type something like "LimeWire" in the Name box. Put the address number of your computer in the Private IP box. Choose Protocol Type - Both. Private Port 6346. Public Port 6346. Under Schedule you can pick the time you want the port open.
THEN CLICK THE APPLY BUTTON.
I'm not sure all these settings are right, but it works.

In System 9:
Go Apple Menu -> File Sharing. That's where the IP Address of your computer will be shown.
LimeWire might allow others to upload without doing these following steps. Maybe LimeWire prefs are used to set the sharing folder and this stuff isn't necessary. I can't remember. I haven't used System 9 and the last version of LimeWire since maybe last Thanksgiving.
Then click the Users & Groups Tab. Highlight Guest, and click the Open button. Choose Show:Sharing and click the button "Allow guests to connect to this computer." You would probably want to uncheck Program Linking.

Also - I forgot this yesterday -
In LineWire Menu -> Preferences -> Advanced -> Firewall. You might want to check the box Force IP