From an interesting thread that covers your issue of the missing option, have you tried connecting via USB cable as you need to do that to enable the wifi option.
And to quote from the thread "I've noticed that you're running OSX 10.5.8. "Sync over WiFi" has an undocumented requirement of OSX 10.6.8. You need to upgrade to OSX 10.6.8 for "Sync over WiFi" to work."
Can you upgrade from 10.6.6 to 10.6.8 easily enough? I don't use a MAC, so unfamiliar with the versions.