Problem with Internet sharing using airport card. HELP ME!!!

I have been trying to set up a wireless network so that I can use Nintendo's wifi with my DS, as I have found out that you can use an airport card, which came with our new iMac, to broadcast a wireless network using airport if you have an Ethernet connection at this website:
http://www.macosxhints.com/article.php?story=20051119155606277

I am however experiencing difficulty setting this up. Every time I reach the step where I turn internet sharing on, it disconnects my airport network, and if I reconnect that, then internet sharing turns itself off. It really is quite infuriating as I have seen many other examples of this working on various websites, while I continue to struggle.

Would someone please help me?

 

no, no!

I DON'T have a wireless network, that's what I need help with.
And I'm getting my net connection from Ethernet and sharing it to airport.

Only problem is that I cannot seem to keep the connection going, it keeps turning itself off and I don't know why.

 

Nup, still having problems.

If anyone would help me, I'd really appreciate it.
The problem is that if I change from the sharing tab to say, firewall or any other menu in systems preferences then internet sharing turns itself off.

And if I turn on internet sharing, then I am forced to leave whatever airport network I'm using.