Its built in to the computer as far as im aware.
You sure? I've just checked and there are a couple of different variations on the Pro 4600 spec - some have wifi, some don't.If yours does it will only be 802.11b. If your wireless router is set to 802.11g only then it won't be able to connect.
If you go into Network Connections you should see Local Area Connection, and Wireless Connection. right-click it and choose "View Wireless Networks".