For the updater for MapSOurce, do you mean the installer from Lambertus’ website? Probably called something like osm_routable_mapsource.exe
If so, run that and follow the instructions to install the maps.
Then open up MapSource, and on the top toolbar there should be a dropdown menu for all of the installed maps, probably with options like MetroGuide, City Navigator etc (if its not there, use View menu → Switch to Product). Click on that and choose “OSM map”.
You can then use the “Map Tool” to select which tiles you want, then do “Send to Device” to transfer them to your eTrex.
If you don’t have an SD-card then there is no point in downloading the gmapsupp.img file. Just download the MapSource installer and use MapSource to put the images on the GPS (or use any other application that can upload maps to the GPS).