Sorry to hear you are having issues. It depands, if you want to use the Curtains Style Start button, you will need to enable it in Curtains, I believe it is enable by default as show in screenshot below (The right picture is on Curtains). But if you wish to change it and use Start 10 start button. You will need to first unselect the Curtain's "Apply style to the Start button" than go to Start10's "What should the Start button look like" drop down menu and select a button. Sometimes you will need to exit the Start10 config windows and reopen it to get that dropdown menu to work.
Stardock Community Assistant.