To add a media file into your Mezzmo library:
Click Add To Library → Add Files toolbar item.
You will be prompted to choose a file (or several files if you wish).
Click
OK after selecting your file(s). You will be prompted to confirm adding the file(s) and, optionally, adding the newly added file(s) into one of your playlists.
Click OK and Mezzmo will add the selected media files into your Mezzmo library.
Tip: To find the files in your Mezzmo library, go to the Recently Added playlist for the type of file (Music, Video, Photos)