This how to assumes that you have already opened gpx or kml file in GPX Viewer. If you haven't opened it yet here is How to open gpx or kml file.

Below are step by step instructions how to add icon for waypoint symbol name.

1. Open left main menu.
2. Tap on Settings button.
3. In Settings tap on Waypoint button.
4. Tap on Icons option. GPX Viewer has already waypoint icons set for some symbol names.
5. Then tap on + icon in bottom right corner.
6. In dialog enter waypoints symbol name.
7. In shown dialog menu tap on Choose icon from library.
8. Choose some icon you like to add as waypoint symbol icon.
9. Go back to main map screen and icons will be changed.

Below is video showing following instructions above: