Since this thread is the first google result for this kind of problem, I will kind of resurrect it and post the solution for my problem, but first of all, a small explanation:
Wi-Fi networks currently at many frequencies, or bands, 2.4 GHz, 5 GHz, and some very special versions in others. I will talk about the most common one: 2.4 GHz.
2.4 GHz networks work with very specific ranges, what we call "channels", they go from channel 1 (2.412 GHz) to channel 14 (2.484 GHz).
Now the important information: USA networks only go from channel 1 to channel 11. Your Mac will only search these ranges. Your Wi-Fi router/access point could be a foreign one, and is staying at channels 12, 13 or 14, outside of your search range.
What you need to do is to enter your router/AP configuration page, and change it to a channel between 1 and 11. Ask a more tech minded person to do it for you. Just by doing it, saving the configuration, restarting the equipment, you should see the network in a minute or two.
By the way, while you or that person is changing the configuration, ask him/her to change the Wi-Fi password, if you never did it before. You will also need to update the password in all your other PCs, videogames, etc that use that network.
PS: As an extra, the best channels to use are 1, 6 or 11, due to interference other channels may have with them (please read http://en.wikipedia.org/wiki/List_of_WLAN_channels). You may find the best one by pressing the Alt/Option key in your keyboard, and at the same time click the Wi-Fi symbol. There you should find an item "Open Wireless Diagnostics...". Clicking it, a new window will show up. Do not press Continue. Go to the Window menu and select Scan (hotkey Command-4).
Please do not use channels other than 1, 6 and 11 (at 2.4 GHz that is), even if the wireless diagnostics tool tells you otherwise. It's a good network neighborhood conduct rule to live by. Hopefully others around you will do the same.