Diamonds and tetrahedrons

I thought it would be, perhaps, easy.  OMG  we cannot seem to do it.  A couple of class 9B students came to see me for help today.  Chemistry.  We looked at the page and the information in the text book and I explained it all.  They have to present diamonds and their makeup and some equations, and what diamonds are used for.  They also have to build a 3D model from a flat diagram.  

I found a better diagram on the internet.  They decided to use nutrella, dried tofu chunks, and toothpicks.  Here are the results, but they are not put together into a cube yet.  Why does the Chemistry text say that each Carbon molecule has 4 covalent bonds, and in the model they have 4, 5 or 6?