When you open the Merge to HDR Pro dialog box, the OK and Cancel buttons are cut off at the bottom of the screen.
Set your monitor resolution higher, to a resolution that allows the OK and Cancel buttons to display. The resolution that allows the buttons to display varies among monitors.
If the dock is at the bottom of your screen, set Dock preferences to Turn Hiding On. You can also move the Dock to another location on your screen. This solution is helpful when you need a lower resolution than the resolution that solves the problem.