If taking the A11 the stop is the first one on the HK island side, after exiting the tunnel. You can buy a ticket at the Citybus booth upon leaving the airport, or put $40 cash bills or coins into the bus.

Evening rush isn't a problem since everyone is coming out of HKG and you'll be going into HKG, also it's the first stop before you hit Central, so the traffic usually doesn't back up that far. The traffic starts after the Shun Tak stop.

BTW, if you need to actually go to Macau, there's a direct ferry from the airport to Macau.
http://cotaijet.com.mo/sailing-schedule/macau/skypier