There are a number of reasons why you might not be able to see your wireless network on the list of available networks from the system menu.
If no networks are shown in the list, your wireless hardware could be turned off, or it may not be working properly. Make sure it is turned on.
You could be out of range of the network. Try moving closer to the wireless base station/router and see if the network appears in the list after a while.
The list of wireless networks takes time to update. If you have just turned on your computer or moved to a different location, wait for a minute or so and then check if the network has appeared in the list.
The network could be hidden. You need to connect in a different way if it is a hidden network.