Sympatico wireless stopped working with my macbook

I've seen a few other threads with problems similar to this but nothing close enough to help me.
Up until a month ago I was having no problem connecting my MacBook to the internet via our family's wireless network. We have Bell Sympatico DSL and are using a Siemens Speedstream 6520 modem/router combo. We have two PCs connected via ethernet and one PC laptop running Linux connected wirelessly, all having no problem connecting.
Then my brother reset the modem and downloaded new software or something, and my MacBook hasn't connected since. My sister-in-law's mac won't connect anymore either.
The Apple store deleted all my system preference files (or whatever they're called -- i'm a new mac user) and that didn't help. I can connect to the Apple store's wireless no problem.
I have spent two or three separate hours on the phone with Bell and they don;t have a clue -- they keep saying to change the modem's security key settings, but we do that and it doesn't work. Another tech guy told me to connect using DHCP, but I was always using PPPoE before.
I can connect using an ethernet cable no problem. BUt when i try to connect using Airport, I get a yellow light and it says that I am connected to our network but airport has a self-assigned IP address and may not be able to connect to the internet.
I am THIS close to selling this mac and going back to PC land -- I got this mac because i thought it would give me less problems than Vista and so far I am not impressed.
Any help would be appreciated. Thanks.

First of all, stop calling the cable company's tech support people--they're worthless. Go to your mac's system preferences, i.e. "apple menu > system preferences…" Then, under "Internet & Network", go to the one that says "Network". Select Airport. Go to "Advanced…", then go to the tab, "TCP/IP"--be sure "Using DHCP" is selected in the IPv4 section. Click "Renew DHCP Lease" just to be sure. Just to make sure, restart your computer. Now if this doesnt work, you need to go into your router's configuration and make sure that it's using DHCP to assign ip addresses to all the computers on your local network. It's probably set to this already unless someone has manually changed it. You may also try resetting that router to its factory default settings (which im sure have DHCP enabled by default).

Thank you!!!

Thanks a bunch -- my brother reset the modem/router to the factory settings (I didn't know he had done it manually -- I don't know much about networks either) and it picked it up right away! Oh thank god...