Go to Interfaces in the Network Wizard window below.
You would see names of interfaces and their types. There will be some buttons present at the bottom of the window having the names of the interfaces that were listed above. Click on the button with the name of such an interface, which is listed as wireless. Most probably, this will be wlan?, where ? is a number.
Click the Wireless button in the Configure network interface window below.
Click the Scan button in the Network Wizard window below to find available network interfaces and select the one you would like to configure.
Click the Encryption button below that provides the desired level of security. Then, in the next window, enter your passkey where it says "Provide a key".
Click the Use This Profile button.
Finally, in the original Configure Network Interface dialog, click "Auto DHCP" (or "Static IP" and enter the IP addresses manually if "Auto DHCP" fails), then enjoy surfing on the internet!