You often cannot see code. Much of MATLAB is provided as compiled code. This gives you efficient code, which of course you want very much. The MathWorks does NOT provide the source code for compiled tools, unless you work there.
Other tools that are written in m-files are viewable. NEVER use edit to examine code, as too often people mistakenly edit the code, then save it without realizing they did so. That will introduce random bugs into the code, a terribly bad thing to do because you will never expect to find an error there, so you will never look for it there. Therefore my recommendation that you never use edit to view code. (In fact, I saw this exact same problem happen quite recently. A user had a supplied function that appeared to be no longer working properly. The problem was they had mistakenly edited the function when looking at it, and then saved it.)
Instead, use the type command. For example, if you want to see how the nlinfit tool (from the stats toolbox) is written, then view it like this:
type nlinfit
If the function was provided in compiled form, then all that will be shown will be the block of help as returned by the help command.
