Help!!!! Garmin File format

[TORNADO]

Have software called Tsunamis '99 on laptop. It allows to plot routes and save them on laptop with the file extension ".rte"
Have Garmin eTrex which connects nicely to this program but no way of uploading plotted routes from Tsunamis to eTrex.
Have downloaded freeware EasyGPS which allows editing/creation waypoints/routes etc etc, EasyGPS connects nicely to Etrex too.
Plan was to save routes made with Tsunamis to laptop then upload from laptop to etrex using EasyGPS program.
However, EasyGPS saves routes with either ".gpx" filename extension or ".loc" filename extension. Anyone else had this problem & solved it?
If anyone understands my translation of all the above and knows how I can convert file formats I will be very grateful and less confused !!