I think I see where the confusion is.
Let's review a couple of concepts. Airport Extreme is Apple's implementation of 802.11G - WiFi. Keep in mind that Airport Extreme is exactly the same thing as a wireless card. And that Airport Express is a wireless device that can be used on a Airport Extreme/WiFi network.
Here is the potentially confusing part. The SB is a network device. As such it is designed to have two-way communication over the network. To use it, (enable it to talk) you first install a free applet on any computer that has a music library that you want to be able to access (eg the one on the 250.)
SLIM has nothing to do with iTunes except that it can read an iTunes Library. You cannot feed an SB with Air Tunes from Air Express - SB doesn't know anything about Air Tunes and has no idea what to do with it. The protocols for two way communication are not there.
Assuming that your PBook has Airport Extreme built-in, you should be able to "talk" directly to a wireless Squeezebox. Effectively the SB is just another wireless node on the Airport network.
If you do not have Airport built-in, you have to buy a 802.11g card to provide your PB with wireless functionality. Then, by adding a router you can mix wired and wireless devices, as well as providing Internet access to all devices. So for instance you can run a wire from your PB to a router, connect to the SB wirelessly through the wireless router, and run a Airport Express to do wireless printing over USB.
(Typically a wireless router includes the capability to access a DSL or cable modem. For instance the SB3 can access the Internet this way for Internet radio)
Also your wireless mileage may vary - it is very environmentally dependent. Does not go through walls and around corners very well.
Hope this helps