So I have a program where I am displaying a treemap (see Ben Fry's Visualizing Data book) of some hierarchical data.  To make a long story short, I have the program opening and zooming further into the hierarchy of the treemap with a LEFT mouse click and zooming out and closing the children of the treemap with a RIGHT mouse click. I am now using the RIGHT click for something else, so I now want that RIGHT click to become a SHIFT and LEFT click to zoom out.

Here is a part of the code that controls this function:
[td]  
boolean mousePressed()
 {
   if (mouseInside())
   {
     if (mouseButton == LEFT)
     {
       parent.zoomIn();
       return true;
     }
     else if (mouseButton == RIGHT)
     {
       if (parent == zoomItem)
       {
         parent.zoomOut();
       }
       else
       {
         parent.hideContents();
       }
       return true;
     }
   }
 return false;
 }
}
[/td]

I tried replacing the else if statement with this:
else if (mouseButton == LEFT && (keyPressed == true && key == CODED && keyCode == SHIFT))

It doesn't work.  It doesn't zoom out at all.  I was wondering if it was because it as a boolean mousePressed?  I got a small program to work with a void mousePressed:
[td]
color fillVal = color(126);

void draw() {
 fill(fillVal);
 rect(25, 25, 50, 50);
}

void mousePressed(){
 if (keyPressed && key == CODED && keyCode == SHIFT && mouseButton == LEFT){
     fillVal = 255;
   } else if (keyPressed && key == CODED && keyCode == CONTROL && mouseButton == LEFT) {
     fillVal = 0;
   }
 } else {
   fillVal = 126;
 }
}
[/td]

Could anyone might see where I am going wrong? Thanks for the help.

Look at your logic (as you describe it):
if (mouseButton == LEFT)
// Do Stuff
else if (mouseButton == LEFT && (keyPressed == true && key == CODED && keyCode == SHIFT))
// Do something else
You will never go into the else if, because when mouseButton == LEFT, you just go in the first statement, never in the second.
Put the key tests into the if (mouseButton == LEFT) condition.