Now open the Network Preferences, select the interface you want to use to connect your GPRS-modem to your Mac and go to the Modem-Tab. Now select the GPRS generic modem.
Now go to the PPP-tab to connect to the desired AccessPoint. Since this is just a fancy name for what is actually the number we are dailing, we just type it in that box. If a username-password are needed, just supply them in the corresponding boxes. Hit "Apply Now" and you are ready to connect. For this you can use the standard ways such as the modem-icon in the menu-bar.
Something else is the mechanism I used to make the modem-script work for almost any modem. It could be that it doesn't work for your particular phone (I didn't see any yet ;-). You could mail me with details of the particular model of phone and I might then fix something or you look into it yourself. Just double-click the modem-file to edit it. The Specification of the modem-file will help you understand what to do.