The charting capabilities in Keynote don't include curve smoothing, so you can't do this directly with the chart tools. However, as a workaround you can use the pen tool to create what you want.
First, create the chart with the points. Then, using the Pen tool (the last item in the Shapes toolbar menu), click on each point of the line in the graph. When finished, press the Escape key. This will create a line that follows the graph.
Select the line, so that you see the red control points at the corners. If you double-click these points, they will become Bezier curve control points, and will turn from a sharp corner to a smooth curve. This should give you the effect you want.
Of course, this isn't mathematically precise curve smoothing, so you don't want to use it for anything but illustrative purposes, but that's what most graphs are used for anyway, especially in presentations.
