When using GSAK you will have to manually transfer the files...
Part One
GSAK -> GPS -> Export to Magellan eXplorist... -> Under "Format required" choose "eXplorist Geocaches" -> Name your .gs file and point it to the desktop -> click Generate and a .gs file will be on the desktop.
Part Two
Make sure your 210 is connected and turned on... It will connect like a "Removable Drive" -> Browse to the Geocache folder and COPY the .gs file from the desktop into the geocache folder of the GPS.
Perhaps this is old news but I had to learn via trial and error.
I loaded 300+ caches this way... I noticed a major slowdown in the processing speed. Also... GSAK will sometimes truncate/abbreviate the cache names. I am still trying to figure this one out. I loaded them through the Cache Manager software and it worked flawlessly.