I just got a new 4th Generation Time Capsule, and the MacBook to which it is attached is running Mountain Lion and Airport Utility 6.10.31. I don't find the ability to set Wide Channels anyplace.  The article by Prince McLean  which you cite, has some great screenshots of how to find that setting, but my version of Airport Utility just doesn't have that setting under Wireless Options.  The article that has those screenshots is from March 31, 2008... nearly five years ago,...