Ok, now that this is in the right forum, can you tell me what kind of broadband service you're using, and what computers other than the PowerBook you want to connect with it?
As for what type of security you should choose, on the PowerBook you'll see it called "WPA-Personal," while on the Belkin it's called "WPA-TKIP." The TKIP option is just as secure as any of the other types of WPA, but it doesn't require a server; it's ideal for home networks. Further, unlike other wireless security schemes, when you type in a passphrase in a WPA device, it is identical to the passphrase you'll type into your computer-any computer that supports WPA.
The Belkin manual has a pretty good description of what to do on the router end. Your PowerBook will see the network (once the new security settings have been saved on the router), and all you'll have to do is type in the passphrase and you should be good to go.