Thank you very much for your swift reaction. I concert the major contours to a kmz file. But how can I select in Google Earth the relevant lines (for example only the 3 meter contour line). As for now, all the major contours (of this huge lake) are in one KMZ file.

My opinion: Export the datas as kmz (Google Earth), preferably only the major contours. Make them visible in color (e.g., white) in GE. Select the relevant lines and copy them directly into the RM Waypoint Manager. They will be inserted as a track. You can now convert them to a route in the RM Waypoint Manager and import the results into the device, or you can export the data as a track and perform the conversion directly in the device.