I am having the exact same problem. The weird thing is, I did have everything set up before and I was able to access the internet. However, I have been playing around with an Airport Express to see if I could play music on my stereo (I can).
Since then, I have not been able to make my original configuration work. That is, following the directions of the article, I can connect to my internet-enabled computer, but I cannot access the internet from the iMac.