Can a Mac's airport card run 802.11 N, G, B or A only?
Maybe this help, you can access to the airport command utility with

sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/sbin/airport


Then, type

sudo airport en1 prefs JoinMode=Strongest JoinModeFallback=KeepLooking


JoinMode = Ranked and JoinModeFallback = Prompt are the defaults, if you want to switch back