"Notepad++" is a good, FREE, open-source XML viewer that can help when you're editing XML settings (.xml) or model (.osim) files.
To get Notepad++, go to this site:
http://notepad-plus.sourceforge.net/uk/download.php
Then click, Download Notepad++ executable files and download and run the "npp.4.8.2.Installer.exe" file.
Additionally, a good way to view .mot, .sto, and .trc is with Excel!