When you go to the URL for the router, you will see what looks like a web site. This is actually the configuration interface on the router. Just like any web site, it has links to other pages, each of which pertain to a particular set of configuration parameters. There should be a link that takes you to a page with the wireless security configuration, which will contain editable fields and menus where you can specify the security settings.
If that's still not clear to you, you can download the manual for the router from the following page:
http://web.belkin.com/support/download/downloaddetails.asp?download=994&lang=1