From Ubiquiti Wiki

VLAN Configuration (thanks to ahooper)

One tip i would give anyone is that if you want to play with the command line VLAN creation you put your unit into router mode as for some reason when it is in bridge mode you cant create the vlans and bind them correctly.

The first process is to destroy the default devices using wlanconfig (that is if you are creating the vlan on the wireless side)

 /sbin/vconfig add <device> <vlan>

will create the relevent vlan on the selected device

you also need to bring the vlan device you have created up

 /sbin/ifconfig <vlan.id> up

Also be sure that whatever you are creating is bound to the correct interface otherwise you just get your tagged packets flowing the wrong way.