SAS generation is controlled through the Solvent Accessible Surface dialog. The generated surface is an iso-surface generated from the union of the Van Der Waals spheres whose radius is increased by the size of the solvent surface. To generate the Van der Waals surface simply use zero as the solvent radius.

To generate the surface simply press the Generate button.

To remove the surface press the Remove button which is enabled only after the surface has been generated.

Molekel 5.1 or newer support mapping of electrostatic potential read from Gaussian .cube grid data; in case you load a molecule from .cube files you'll have the option to color-code Solvent Accessible Surfaces with values read from grid data, in this case a Map MEP(From Grid Data) label will appear close to the map MEP checkbox.