I don't believe any Garmin software will work to transfer data to your GPS because it's serial. Garmin states that fact on their website.
I've got almost the same setup as you. OSX 10.5.6 and the USB>Serial is Keyspan.
I transfer data using MacCache as it 'sees' the gps, and as BlueDamsel mentioned, you need to select the correct serial device amongst those offered. Make sure the Keyspan driver is run and the gps connected before you start MacCache.
Thank you. I'm beginning to believe you're correct about the serial thing. My Mac does not have a serial port, hence I'm am using the usb adapter. Unfortunately I have been at this for about a week now and I still can not get it to work. Oh well, I guess I can always try to talk my way into a new gps!
Thank you to all that have attempted to help thus far.
It just doesn't work using the Garmin software.
It works using MacCache or GPSBabel with the KeySpan usb>serial adapter.