What's going wrong?
Were you trying the POIEdit program or a different one?
I'm used to POI Edit so let's try to lead you through it with that one....
1. Download and install POI Edit from the link in post #3
2. Once it is up and running, you should be presented with a pretty blank screen.
3. Go to the "File" menu at the top and select "Open"
4. A normal file selection window should open, use it to find the .gpx file, wherever you have it saved on your PC.
5. Select it with the mouse and click on "Open"
6. You should now see a list addresses of of the "waypoints" in the .gpx file on the right half of the screen
7. Clicking on any one of them should bring up some sort of map of the location on the left half of the screen.
(but if you want to use Google maps, you'll need to do a bit of tinkering as per
THIS TOPIC. - however getting the Google maps bit working is NOT necessary for the job you want to do.
8. Go to the "File" menu again and select "Save as..."
9. Another file selection window should open up. You need to go to the BOTTOM of that window and where it says "Save as type..." use the drop down list to select "TomTom Itinerary (.itn)" - it's the second item in the list.
10. Type a name for the file AND choose a location where you want to save it (in the normal Windows way)
11. That's it!
Connect up the TomTom, copy that new .itn file into the "itinerary" folder on the TomTom and you should be able to select it and load it when you go to Itinerary Planning on the unit.