Thought this might be of benefit to others searching for a similar solution. I was disappointed to discover that Media Manager does not exist for Mac, so went about looking for a substitute playlist creator. I saw the post linking to the iTunes Applescript; however, my music sits on a separate server and I do not use iTunes to access it, so that was a non-starter.
I downloaded a number of applications that claimed to be able to create M3U playlist files with mixed success. I then realized that an application I already had installed on my Mac would do the trick: VLC. (http://www.videolan.org/)
Plug your Sansa Clip into the computer and open VLC. Open Finder and navigate to the Sansa Clip drive. Now you can drag and drop items from the Finder into the VLC media window. If you drag folders, you will need to double-click on them to populate the list with items within the folder. You can add and delete items at will. When you have the playlist as you like it, go to File, Save Playlist and save to the main MUSIC folder on the Sansa Clip.
Then, one last step because the Sansa uses backslashes and the Mac uses forward slashses to separate folders: in the Finder, find the M3U file you just created. Right-click and choose Open With -> TextEdit.app. Now do a Find and Replace (Command-Option F) and replace all / characters with \ characters.
Hope this helps…