Wifi config on a Multitech mLinux Conduit

Can anyone help me configure a Multitech mLinux Conduit to work over wifi?

I have set up my interfaces file:

iface wlan0 inet dhcp
wpa-conf /var/config/wpa_supplicant.conf
wpa-driver nl80211

And I have set up /var/config/wpa_supplicant.conf:

ctrl_interface=/var/run/wpa_supplicant
ctrl_interface_group=0
update_config=1

network={
key_mgmt=WPA-PSK
ssid="PlusnetWireless9xxxxx"
scan_ssid=1
psk="xxxxxxxxxx"
}
…which I think is correct. I’ve got a little USB wifi thingy that works fine on a Raspberry Pi and plugged it into the USB port on the back of the Conduit.

When I try to start up the wifi – ifup wlan0 – I get:

Successfully initialized wpa_supplicant
rfkill: Cannot open RFKILL control device
Could not read interface wlan0 flags: No such device
nl80211: Driver does not support authentication/association or connect commands
Could not read interface wlan0 flags: No such device
wlan0: Failed to initialize driver interface
ifconfig: SIOCGIFFLAGS: No such device

Grateful for any help.