Unfortunately ADB connected Wacom tablets are not supported under MacOSX. This is an ongoing argument on Wacom's support forums. The long and short of it is that legacy ADB support under OSX uses a software workaround that means that the bandwidth is too low for tablets, and that even if Wacom could make a driver then it would be slow and laggy.
To their credit, Wacom don't expect owners of large ADB tablets to just give them up. For their largest sizes that have a seperate logic board and sensor board inside, they are offering a free upgrade to USB. However, if you don't have one of these HUGE tablets then you won't be able to get the upgrade and will have to update to a USB tablet. :-(