Desktop -> Admin (or System) ->Network
Then you will be asked to give your root password.
Next you will find several network card named eth0, eth1, eth2.....One of these is the wireless card. Go on "properties" and insert your parameters (IP, gateway, netmask, ESSID, wep/wpa key).
Type "/etc/init.d/dhcpd restart" and everything should be working. If not, you maybe need to install appropriate modules depending on the wireless card you have