Alas, it is not possible to display a real button unless it’s with a string response edit field. But you can always display an image of a button and create a response that happens to overlap that image. Then:
- In the Event Editor's [B]Input[/B] tab, set the event to end [B]After a correct response from the participant[/B].
- In the Event Editor's [B]Correct Response[/B] tab, set the correct response to [B]One or more of the following[/B] and turn on the blue checkbox that is next to the response that you created (the one that overlaps the button).
It it helps, most of the experiments that I've helped users with simply ask the participant to "press any key to continue" or "click on the mouse button to continue" without regard to which key was pressed or where the mouse button was clicked.