I have been having a terrible time getting the standard USB-to-Serial driver used for MCT to work under Panther, although it used to work under 10.2. Part of the story has been detailed here at OSXHints in the discussion boards. The driver typically used can be found here [mct.com].
I found a quick fix for what appears to be a permissions problem. I have tried it with two Garmin GPS receivers, and both appear to now work. The quick fix is as follows:
% sudo chmod -R og-w /System/Library/Extensions/USBSerialDriver.kext % chown -R root:wheel/System/Library/Extensions/USBSerialDriver.kext[robg adds: I can't confirm this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040205073630276