Which offset tool are you using, there are three of them, distance, point, and face edge? Select the curve/s to be offset and click on the side you want the offset to be on, then set the distance in the data window box. If the distance is set before you click, then the offset will be at the wanted distance. A Smart Polygon will offset all four sides whereas with a Single Line rectangle you can select one or more sides to be offset.
See page 264 in the User Guide.