WPA on Raspberry Pi(rateBox) running PirateBox 1.0.2 OS

I'd like to set up a password for the WiFi network my PirateBox generates. I'm using the PirateBox installation image based in Arch, a distro I have no familiarity with. Through tutorials I managed to get everything set up and working, but now the only tutorials I see for setting up encryption are for Raspbian (my system has no /etc/network/interfaces file to edit).

How do I do this on the Arch version of PirateBox? (If it's of any consequence, I'm using the Edimax EW-7811Un)

Thanks!
SG