You are missing the second step in the Tutorial... Get the mesh face THEN make it a Solid - to do this draw some vertical lines down from the four corners and then some horizontals the form a faced solid object. Move the flat bottom face up so the minimum height corner is quite small otherwise you'll get lots of rectangular slices at the base! A flat-ish cuboid with a 'wavy top' if you will... NOW group it all and use the Slicer tool on that group. It should now 'slice' OK... Slicer won't make slices of a single surface mesh because there is literally nothing to slice! There is Slicer's stablemate 'ContourMaker' that will make contours lines through the mesh... but these aren't 'slices'