There is a checkbox at the top of the Layers tab in Snap Art 3 labeled "Activate tab when drawing". If that is on then the Layers tab will pop to the front whenever you draw on the layer mask in the preview area. That is usually helpful because the Layers tab helps you control how the layer mask affects the image.
When I say "draw", I mean changing the shape of the layer mask, not actually drawing brush strokes. When you scribble in the layer mask you are just showing Snap Art 3 where you want more or less detail. Individual paint stroke placement is handled by Snap Art's super cyber brain.
To learn all about layers in Snap Art 3, watch the Getting Started and Portrait videos on our tutorials page.