A flow step is a form. Steps are added to the flow by dragging one of your existing forms displayed in the palette, the New Form or the Summary and dropping it into the flow work area.
The Form Viewer Control is used to allow a generated PDF to be viewed as part of a form activity in a flow. In this version of frevvo, this control only appears on the Forms palette when you click the icon to edit an activity in the flow designer. Refer to this documentation for the details.