Have you got ythe polygon button checked in the create path dialogue?

If so, uncheck it. Now after designing the path you want, change the tool from Design to Edit (there are three radio buttons for the path tool in your toolbox). Now go to the point you want to smooth out, click and drag. This pulls out a handle, which controls the curvature of the path in one direction. Move this until you get a smooth enough curve. Then click and drag the point again in the other direction. This should pull out a second handle to smoth the corner in the other direction. It's a bit fidly to start with but it gets easier with a little practice.