http://msdn.microsoft.com/en-us/library/windows/desktop/dd562624(v=vs.85).aspx
Halfway down the page it lists:
/Playlist PlaylistName
Open the Player and play the specified playlist.
Launch the program with QProcess
and specify the arguments.
http://qt-project.org/doc/qt-4.8/qprocess.html
Hope that helps.
EDIT: If you still want to use the WMP API, you could look into:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd563405(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/windows/desktop/dd563242(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/windows/desktop/dd563547(v=vs.85).aspx