I'll echo what skoehler said about checking that the Fantom driver is installed.
Yes, you could indeed try to install leJOS separately, but you shouldn't have to do that to use Enchanting.
I vaguely remember an issue I ran into where a USB device (a SMART Board, as I recall) was detected as a network interface under OS X -- may've been 10.6. My google fu is failing me on the issue. Are there any software updates to apply? Are you running the latest on the 10.6 line (10.6.8, IIRC)?
It might not hurt to try LEGO's official NXT-G software. You can download it for free
. I don't think you'll have any better luck if the NXT itself shows up as a network device, but it won't hurt to try.
I vaguely wonder if you can go to System Preferences, choose Networking, find the NXT that is misidentified as a modem in the list, and hit the minus button at the bottom to delete it. Maybe reboot and maybe run disk utility and repair permissions and re-install the fantom driver and reboot again? (I don't see an obvious way to uninstall the fantom driver. I could think of a tedious way to do it, but I don't know if it would help.)
If none of this helps and you see something like an error message telling you a modem was identified, could you take down what it says or take a screenshot?