Successfully downloading music onto Sansa c250

I was given a refurbished Sansa c250.  I have no manual.  Have followed all the seemingly logical steps to download music onto the mp3 player.  When it is connected to my computer, the files I have downloaded are listed.  However when the device is not plugged into the computer, only a handful of songs are listed and available.  I have formated all files to mp3...I'm not sure what I'm missing in the process.  Has someone dealt with this problem before?  Is there a manual online I cam read through (I've looked and keep finding help sites)?