How can I activate the built-in visualizations for WMP as well as download new ones from the Internet?
You can activate a visualization when you play an audio file or a video file that doesn't have a video stream. To activate visualizations, play a track, then switch to Now Playing (use the image as guidance) then right click inside the windows and select Visualizations.
Download visualizations: http://windows.microsoft.com/en-us/windows/windows-media-player-visualizations