How are you trying to put the music on? Are you dragging & dropping the files from your computer to the Music file on the Fuze? That’s the easiest way. You should be able to do this in either MTP or MSC mode.
I prefer MSC mode for it’s straight-forward simplicity and reliability. In this mode, your computer sees your player as just another flash storage device. MTP mode is more complicated and necessary only if you intend to use Rhapsody, or Napster or any subscription-type service. Also DRM-encrypted Audio books (Overdrive)
Remember though, that to avoid confusion you should pick one or the other and stick with it. Files put on in one mode won’t be visible on your computer if the player is connected in the other mode.
Explain what you’re doing and what you mean when you say MSC only lets you put a few songs on.
Have you read the User’s Guide, or checked out any of the “How-To” videos?