the only guidance that I can give you is to repeat what I already said:
If the chipset in the serial to USB adapter does not allow you to 'see' the tablet, then you will need to try a different brand of adapter.
Keyspan, or Iogear, maybe Belkin - one of those might work.
One of those brands should work.
It's not that your present adapter isn't "good enough", but that the built-in chipset in the adapter is not compatible with the comm chip in the Wacom tablet.
Or, you're just on a fool's errand, and you won't get this to work on a Mac.
You said that you had it working on an older PC. I see reports where people using newer PCs with Windows 7 couldn't get the older Wacom tablets with serial connection to work, but, of course, that is sometimes a 64-bit driver issue, which is its own puzzle on Windows.
Or, you can try googling to a site that might have more direct answers for you...
Here's a Wacom support article that talks about the challenges of using serial devices on OS X, and additionally, that tablets add to that challenge, as tablets use a non-standard connection protocol.
Serendipity is a lucky guess !