To select a layer that you want to edit, just click on it in the layer list (or use the up and down arrows). You can change the layer order by dragging their name. The order affects the way image layers are displayed on top of each other; it does not affect the sound and camera layers. Layer properties, such as their name, can be changed by double-clicking on the name (see above).
Layers can be activated or deactivated by clicking the circle at the very left of their icon. A deactivated image layer is hidden from the canvas view.
If the currently selected layer is an image layer, it is shown in the canvas view at full opacity. By default, the other image layers are semi-transparent, to help you focus on the selected layer. However, if you wish to see all the image layers at full opacity (as they will appear in the final rendering), press the circle placed by itself above all the layers to turn it black. If on the contrary, you want to concentrate on the selected layer and hide all the other layers, press again the circle to turn reduce the opacity of all layers to none (with the exception of the layer you are currently working in).