Click on a function button to acitvate it. Single argument functions operate on the x register. It is not necessary to click on [ E ] (after entering digits) before clicking on a function.
Some functions will beep if they are attempted when illegal values are in the used registers. In general, however, there is very little error check-ing, so be careful when clicking.
Pressing the option key while the cursor is on a button will cause a short help message about the operation of that button to appear within the register area. Release the option key to restore the register values.