OK, this is what I would recommend:
Go to this website: http://garmin.na1400.info/routable.php
Select the tiles for the area you want, enter your email address, and click “Build maps”.
You will be emailed a link with several options to download the map. The one you want is the “osm_routable_gmapsupp.zip” file, so download it, and unzip the gmapsupp.img file.
Then you need to copy this file to the microSD card.
You can do this by putting the microSD card in a USB card reader, or you can connect the eTrex to your PC with the USB cable, and put it in USB Mass Storage mode (on Main menu → Setup → Interface).
Then create a folder on the card called “Garmin”, and copy the gmapsupp.img file to this folder.

Then put the card in the eTrex, and switch it on, and map should be loaded. You should be able to scroll to the right place, and zoom in on it, to check its there.