Can you let me know which model eTrex you're using, and does it need a special adapter to connect to the Mac USB? I read that it has a serial output and needs something to connect. Do you have the problem? And, can you describe the series of steps you use to load into lightroom and sync with the import.
My LR4 Geotagging Workflow:
1. Garmin eTrex Legened HCX (hanging off photo-pack) turned on at location
2. Load photos into LR4
3. Connect Garmin eTrex to Mac via USB to Mini-USB cable.
4. Launch free Garmin 'LoadMy Tracks' application - select 'Tracks' and click on 'Acquire' button (I created a folder called 'GPS Tracks' in 'Documents' which it now remembers each time)
5. In LR4 Map Module click on the track icon and select 'Load Tracklog...' - direct it to the directory/file you chose above.
6. Select all your images - click on the track icon and select 'Auto-Tag Photos' - it will complete this in a few seconds as opposed to the 20 minutes it used to take with all the 3rd party solutions I tried before.