Assuming you have the GPX file on your computer, just connect the Dakota and you may have to follow a prompt (on the Dakota screen) to put it into 'USB mode' or some such, so that it appears as a removable drive (or 2, if you have a mSD card fitted). Then copy the GPX file into the /Garmin/GPX/ folder (either on the device or the card, either should work).
Then (as with any USB drive) take care to 'unmount' or 'eject' the removable drive before disconnecting.