Use the Insert Token tool to insert the <tdbutton> token into your layout. This can be configured to act as any of the available action buttons.
For example, using the default layout you could edit the "Blank" cell near the bottom of the layout. Then edit the Contents. Then press the Insert Token button. Select tdButton, and select the various options you'd like. Press OK to insert the token into the Contents, then press OK to finish editing the Contents, then press OK once more to close the Cell Properties dialog. You should now see a button in the Game window.
You can also load the "Black and Green with Buttons" layout as an example. Press the "Built-in" button on the Layout tab and select the "Black and Green with Buttons" layout.