Create seperate 2.4 & 5GHz WiFi networks on the Extreme, name them differently e.g. Home24 & Home5
I believe the AC operates on 5Ghz band only. You need to make sure you connect to that.
My rMBP Haswell is connected to to 802.11n instead of 802.11ac using the latest Airport extreme. Any idea how to change this to 802.11ac?
Wifi can only go as fast as the slowest device connected. So it you have a wireless G device connected the computer will never get AC.
This is not correct for modern MIMO wireless networks. It was more a problem when G networks first came out and they only had one antenna. Current MIMO wireless routers drop one of their antennas to the A/B/G protocol and keep the other one or two antennas operating at the N/AC protocol. There are even some MIMO G wireless routers that are also not effected by this limitation.