I have recently got a Garmin GPS - a 255W Car Navigator, but you can use it to find a lat/long as follows...
Make sure you have the Garmin Browser plug in installed. Then go to
http://maps.google.com/ and enter the coordinates there to get the map.
Then with the GPS connected to the computer by USB cable, click on the pin on the "Send" link on the top right of the Google map and in the window that opens, GPS should show up on the left side along with eMail and phone.
Click on the GPS tab there and you can send that location to the Garmin where it will be saved as a Favourite for you to later navigate to.
It doesn't matter if you started out entering a street address into Google or a lat/long - the end result is the same in that you can send it to your Garmin as a favourite place and then navigate to it with the GPS.
Roger