If you're wanting simply to make adjustments to what S3D automatically creates, use the Customise Support Structure window. Click the big Generate Automatic Supports button. This will create lots of columns indicating the centre of the area where support will be automatically generated.
You'll then be able to switch on and off the Add.... and Remove.... functions by clicking on the respective buttons. If adding, move the cursor to drag a new support column into place and click to add it. If deleting, use the cursor to highlight a column to delete and click.