Microsoft Excel can read .csv files, but so can many other programs. When Excel is installed on your computer, it automatically associates the .csv extension with Excel. This is why the files show with an Excel icon.
If you look closely at the icon, you'll notice it's slightly different than the icon that's normally used for Excel spreadsheets / workbooks.
Even though these models have a SD card slot, they cannot read custom POI data from a SD card.
Several users have reported confirmation of this from Garmin technical support reps. The attached discussion topics include some examples of email responses from Garmin.
Garmin's POI Loader software can read .gpx (GPS exchange format) and .csv (Garmin comma separated values) files.
POI Loader will also read Icon files (.bmp) and Sound files (.mp3 and .wav).
Note that if you use .wav files for sound, you must have sox.exe (see http://www.poi-factory.com/node/25730 for details) in the same directory that contains POI Loader.exe.
nuvi 200, 200w, 205, 205w, 250, 250w, 255, 255w
nuvi 260, 260w, 265t, 265wt, 270, 275t
nuvi 300, 310, 350, 360, 370
nuvi 500, 550
nuvi 600, 610, 650, 660, 670, 680
nuvi 750, 755t, 760, 765t, 770, 775t, 780, 785t
nuvi 850, 855, 880, 885t
StreetPilot c310, c320, c330, c340, c510, c530, c550, c580
StreetPilot i2, i3, i5