I have recently bought a Garmin eTrex GPS receiver to use for geocaching.
It has a "Memory-Map" USB to Serial cable, and a windows-only trial mapping program, which gave me enough clues too work out how to write a command line fist for windows and then for linux.
However I wish to also do it with my MacBook ( MacOS X 10.6.2 ) - the computer I take away on holidays for photography and now geocaching.

I have a Windows/Linux dual-booted desktop, and a Linux - Ubuntu 8.04 laptop. Both of these can read and write to the Garmin eTrex to upload or download latitude and longitude co-ordinates.
I use the terminal program gpsbabel like this :-

gpsbabel.exe -i geo -f geocaching.loc -o garmin -F com3: - for Windows

gpsbabel -i geo -f geocaching.loc -o garmin -F /dev/ttyUSB0 - for Linux

and would like to do something like this on my mac-book :-

/sw/bin/gpsbabel -i geo -f geocaching.loc -o garmin -F /devicename

BUT it says "cant open serial port 'devicename' "

I found the Linux "/dev/tyUSB0" by looking in the /dev directory and seeing a few things that looked useful.
However I can't see anything on my MacBook /dev that looks useful.

The MacBook "System Profiler > USB Tree" correctly identifies the USB/Serial cable as long as it is plugged in, but I guess there needs to be some sort of device driver with a link in /dev.

Can anybody help me please.

The Geocaching website says to use on a mac one of the GUI programs -

Garmin BaseCamp
Garmin RoadTrip
GPSContact
GPSBabel

I have all of these working, and all give the message "no Garmin PS was found".

GPSContact offers a choice of USB,Bluetooth-PDS-Sync,Bluetooth-Modem for the device to be written to, and there are tty.Bluetooth-PDS-Sync and tty.Bluetoth-Modem in the /dev directory.
The other 3 have an Input and Output icon with nothing else specified.