I have never done what you want to do and I don’t have one of these GPS’. I do work with georeferenced images and maps though so I had a bit of a look. The following looks like it should work. If not at least your looking in the right places.
Go to that ICC page – zoom in to the area you want, select the map scale you want in the right hand side. Click on the map you want and a pop up window appears download the .sid file. These are georefereced (ie in the correct geographic coordinates) images of the maps
Now use one of the tools from (you’ll have to copy and paste as I can’t get the link to work)
“http://garminoregon.wikispaces.com/Custom+Maps#FAQ-CM5.)What other tools can I use to make Garmin Custom Maps?”
to convert the sid file to a file type that will go in the GPS.