This code first gets the current mouse state, and then checks to see if in the current state, the button was pressed, and if it was released in the old state (from the last update). This is equivalent to a button press, since it was up before and down now. If it was pressed, then do whatever you want it to do, and then finally, set the old mouse state to be this current state. By the time the next update comes around, this will be the old mouse state.
Also, while we are at it, let's see how to get the mouse position. This is also quite easy. The MouseState class has properties that are defined for the mouse position. So we need to do something like the following: