Ugg... Need help with D-Link router.

[Unorthodox]

Pardon my english, but I'm having a brain fart and having a hard time conveying ideas.
But I'll try.

Here's the scenario:
I have an AirPort Extreme base station connected to my high-speed internet modem.

I recently acquired a really old iMac. The original iMac, actually.
What I want to do is connect my spare D-Link router to the old iMac so I can access the internet.

How do I get the D-link to join the network created by the AirPort Extreme, and send the signal via ethernet to the the old iMac.

I figured out how to get to the setup wizard by typing "192.168.0.1" into my browser.
That's as far as I got.....
What do I do next?

I'm going to bed....