This guy has the answer:
If you have the cert then this should work for you.
This is how I did it.
First I grabbed the public certificate that the RADIUS server was offering and saved it as a PEM file.
sudo mkdir /etc/certs
sudo cp radius.pem /etc/certs/
Then I edited /etc/wpa_supplicant/wpa_supplicant.conf
then edited /etc/network/interfaces
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp
pre-up wpa_supplicant -B -Dwext -i wlan0 -c/etc/wpa_supplicant/wpa_supplicant.conf
post-down killall -q wpa_supplicant
It sounds simple but it took me a while to crack it. I am also the
person who set up the RADIUS server (FreeRADIUS) so I have some inside
knowledge. I hope this helps.