Z,
A few things:
1. I'm assuming upgrading your phone means the new one has a modem in it. Without a modem, you cannot access the internet and the Macbook does not have a modem in it.
2. Now you need to figure out how to enable the modem feature of the phone (if it needs to be enabled). This is a question for AT&T.
3. What you need next is an ISP to provide you with dial-up internet service. I'm not sure if AT&T offers this, but it's definitely something you want to ask them. Alternately, if your ISP at 'home' has dial-up access you may want to find out what those numbers are.
4. Once you've confirmed 1 and 2 above, and inquire about number 3, we can work on getting Macbook connectivity. Curious, though: where are you seeing "in fact the category of using phone as a modem is not allowing me to check it off"? On the Mac or the phone? It is possible that specific software is required from Motorola. If that's the case, it would obviously need to be Mac-compatible.
All in all, I'd say all of your queries need to be directed to your carrier, AT&T.
Let me know how it goes.