Do you have any other devices which are connecting to your WiFi network? Are they working at the same physical location you are trying to setup this computer?

Look at the settings of one of the working systems to see what you need to set on system that can't connect.

The wireless Bridge or wireless Router (called an Access Point {AP} for either) may also require a given type of 802.11 service (A, B, G or N) or a given band in the case of 802.11n (2.4 or 5.0GHz) for your systems to connect to it. Make sure your system can access the AP's service type settings.

Some older MacBook Pro's can't access 802.11n networks without a firmware upgrade, or access the 2.4 or 5.0GHz bands

How to tell if 802.11n-enabling software is installed:

http://support.apple.com/kb/HT2447