The right-hand part of Photo Lab is the Post-processing area. When you first start Photo Lab, it will show a few pre-defined settings to process the image (Make brighter, Make darker, Neutral, etc).
At the bottom of this area you'll see an Advanced button (use the mouse scroll down if you don't see it), click on the Advanced button to find features that let you put that extra effort into your photos by fine-tuning them or adding different effects:
All post processing is performed on rendered images, so make sure that the photo you wish to work with is displayed in preview area and has a green frame around it.
Below, you will find links to each of the features in the Post processing area. Each feature is explained step-by-step, and examples are provided on how to use it. It is important to remember that all the different features interact to create a photorealistic image.
From the Advanced mode, you can also switch to the Simple mode from the orange triangle drop-down menu, or from the bottom of the Post processing section: