Well I hope you like it! Ok - I guessed you were in the UK. Your USB cable connection is not going to work with the Mac. The PowerBook comes with ethernet built in, so no problems there - you can just connect it straight into the cable modem until you get your wireless sorted out.
Now the relevance of the MAC / Mac issue. You are going to have to tell your ISP (NTL? Telewest?) the MAC address of your Mac. (to my critics: notice how the caps make the distinction). Kimberly - a MAC (Media Access Control) address is a unique number that is associated with your computer's network ports. The cabled network socket in the PowerBook has a unique MAC address, and you must tell your ISP this number before the computer will work using your cable modem. (Cable companies in the UK usually allow 5 MAC addresses to be registered; note "MAC" has nothing to do with "Mac" - every networked device that uses Ethernet has a MAC address) ISP's do this to restrict the number of computers that you connect to their network.
The easiest way to find out the MAC address of a PowerBook is this:
Open up the system preferences (it looks like a white square with an Apple logo on it in the dock along the bottom of the screen).
Click "Network"
If you see a list, pick "Built in ethernet", then "configure".
Click the button in the row that says "Ethernet"
You should see a number called "Ethernet ID:" - it will look something like this 00:0a:95:12:34:56 - this is a MAC address, and it is the number you will need to tell your ISP.
Note that when you go wireless, the Airport interface will have a different MAC address - you can get this in the same way, but choosing "Airport" rather than "Built-in Ethernet".
Now - how confusing would this have been if you didn't know the difference between MAC and Mac?