Evening guys. Been having the same problem with my iPad. Read all the stuff, but, being a dumb buisness guy, can't deal with much of it. I then took my iphone and while showing full-on wi fi bars, still not connecting to the imac.
But both ipad and iphone connect to an airport in another part of the house no problem. So this is a local imac issue, and, must be something to do with the network currently set up to sync the imac with the little guys.
Therefore, I thought, why not just kill it, and set up a new network. In Preferences/Network, there is a friendly assist me button that let's you do just that in a few keystrokes. And just to make sure the old network wasn't going to get back in the door, I gave the new one a specific channel number, instead of automatic like before.
Problem solved. for me anyway. Both my ipad and iphone are communicating over their new WiFi network happily as before. Perhaps it helps someone out there too.
Cheers.