Hello, I’ve had the same problems with my daughters Sansa Clip and itunes/ my iMac. After quite a few hours :womanmad: I have finally sussed it.
Here are my hot tips from a Mac newbie- open up your itunes music library; plug in clip and open up music file from it; highlight the album/ song to be put onto the clip; go to “file” drop down in itunes and click “show in finder” which will bring up a new window with the song(s) in it. Highlight each song and in the advanced drop down “create mp3 version”. You should then have 2 versions of the song- just drag & drop the mp3 version into your music file on the sansa clip. Repeat as necessary. It seems a bit long winded at first, but this way you don’t have to download any media players, and if I can understand it anyone can :womanvery-happy: HTH.