To assign to a custom button...

Right click an empty space around your toolbars, click Customize. Click the Commands tab and in the left pane, scroll down to Macros. Click and drag the smiley face in the right pane over to your toolbar (wherever you want it to go), then close the Customize window. When you click on your smiley face button for the first time, it will as you what macro you want to assign to it. Given you've put one of the above routines in a macro, type in the path or select it from the list and click OK. You should be set! So everytime you click that button (w/ the above code(s)), you should be toggling the Calculation mode.