Here's a brief explanation.
In Mapsource on your PC, open the GPX file. Once it's open the Waypoints tab should show the number of waypoints. If there are none, then none were in the GPX file. Similarly, look at the Routes tab and it will show the number of routes. If there are some, click on the tab and then click on the route and it will show on the screen for you.
If you find there are routes and waypoints and you want them on the GPS, connect the GPS with your connector cable, power it up, and then click on the little GPS icon at the top center of the Mapsource screen (it should be the right hand one with the arrow pointing down).
Once the waypoints/routes have been transferred, you have two choices. If you want to look at a waypoint, press the find button, use the slider scroll bar to find the stored waypoints, and click on the one you want to see. If you want to look at a route, press the ROUTE button and select the list and then the route you want to see.
I hope this helps.