How to manually assign a fixed IP address to Raspberry Pi
While WICD network Manager is an excellent choice for configuring wireless networking with allocation of an fixed IP address, this can be achieved manually in Linux by editing the relevant \etc\network\interfaces file as shown below.
As shown above replace the text inside /etc/network/interfaces file making sure you chnage the wpa-ssid value with your wireless access point name and wpa-psk value with your wireless access password.
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet static