It's under your wlan's settings (Config -> Wlan -> hit edit on your wlan). You'll find a checkbox for 802.11r (FT Roaming) and one for 802.11k neighbor lists — it's best to enable both or disable both.
Note that turning on 802.11r may cause ancient clients (e.g. more than 3 year old OS'es) to refuse to connect to your network. If you have to support such clients, you may need to clone your wlan so that you have one that's 802.11k/r capable and one that is not.