OS X 10.6.5 supports NEF files of the D3100, so you don't need any software to view them or do basic edits. Preview, for instance, can open RAW files just fine. However, unless you use specialized software such as Aperture, Lightroom, Adobe Camera RAW or Nikon's own RAW converter, you won't be able to make use of the extra information contained in RAW files. If you use iPhoto, I doubt shooting RAW will be very useful.
RAW files are `digital negatives,' the `raw' data from the image sensor. If you know what you're doing, it can really be beneficial to shoot RAW. If you're a beginner, I'd start shooting jpg and focus on getting the exposure right first. Then you can increase complexity step-by-step. As a rule of thumb (which I made up myself

), if you're still using the Scene Modes (landscape, night portrait, etc.), there is no use shooting RAW. I'd start by learning how to use A, S and M as well as changing the ISO consciously.