Which etrex are you using? Since you mention USB I would assume its not an older model, yellow, or Yellow H.

I use GSAK but you could just as easily use Mapsource or easygps for free.

Another option that’s much easier than entering coordinates by hand is using the Send to GPS button located on every cache page. It only works with garmin units that have a USB connection, the only thing you need is to install the communicator plugin found here:

http://www8.garmin.com/products/communicator/

As far as I understand it the Yellow etrex won’t work with the send to GPS button on the cache page. It has to be a USB garmin GPS, you can’t use a USB to serial adapter(the etrex yellow only has a serial connection).

First thing I Would say to try is in GSAK click on the GPS drop down menu, and select send waypoints. in the box that pops up there is a GPSr setup button on the bottom. In here you set it for garmin, and since you are using a serial GPS and probably a USB to serial adapter uncheck the USB box if checked.