Sounds like the resolution of your monitor is making the menu appear a bit too large for your screenspace.If you cannot resize the window or move it to make the button visible, you should be able to work around it for now by temporarily changing your resolution or scaling settings before you open that menu.
Open the settings menu and search for display, then select the option to change the resolution.
Here you can adjust the Display Resolution or Scaling Settings on this screen. I would reccomend starting by adusting the scaling down to see if it makes the popup fit properly.
After you have saved your DYNAMIXEL Wizard settings you can set the scaling and resolution back to their previous settings.