Like Sith said you need the gamebit to open NES games.  This can be found at the website I mentioned earlier, just search for the word Nintendo.  The problem is more than likely just a little plastic piece that holds the gameboard in place.  Its probably not too big of a deal unless both sides are broke and your board moves around freely.  You can use a small drop of super glue to reattach the plastic piece once you get the cartridge open if you really want to, but like I said, its only really necessary if both sides broken.