The tutorial video does not cover mirroring, and only covers the basics for new users. The mirror feature can be found in the Guides menu. The Guides button is located on the left side of your palette, near the top. Mirror is at the bottom of the Guides menu.
If you have already started sculpting a model, and would like to mirror it, that’s a little tricky. You could try copying and pasting it while using Grid snapping (also located on the Guides menu).
Hope that’s what you were looking for!