I work in CS4 or whatever, so I dont know "elements" but you can probably hack it. Create the size and shape that you want using the rounded rectangle tool, dont worry about fill color. Rasterize the layer. And then select the center of the rectangle and either modify, contract by 1 or 2 pixels, and then delete to create the outline only (then do color overlay for whatever outline color you want), or create a new layer, fill the selected new layer with what ever color you want the outline to be, contract, delete, etc. And now you have an outline of a rounded rectangle without any fill.If you need a vector version you may need different software.