You use the Pen tool, found at the bottom of the Shapes dropdown menu, to draw a curve. Select the pen, then click on the slide to put down the starting point of your curve. Next, click
and hold where you want your curve to end, and drag away from the endpoint that is created. You should see two grey "handles" sprout from that endpoint, with your cursor at the end of one of them. The point you have created is a Bezier control point, and the handles control the angle and depth of the curve. Hit Escape to finish. (Your curve will likely be filled -- you can adjust this in the Graphic Inspector.)
One Bezier point will produce a curve that only curves one direction. To create a more complex curve with multiple inflections, you will need to add more points. Select your curve, and in the Format menu, under the Shape flyout menu, choose "Make Editable". You should now see your two endpoints in red. If you hold down the Option key and click on the path, you will add another control point. Double-clicking on the point will toggle it from a corner point to a Bezier control point.
Bezier curves can seem complicated at first, but this is an extremely powerful way of creating arbitrary shapes, including complex curves.
(Note that the paths in the Motion Action are also Bezier curves.)
