The control in the screenshot and the demo that you provided a link to is RadTimeline, not RadTimeBar. That is why, I will give you the information about RadTimeline.
The element that you want to add is an annotation. You can read about annotation support in RadTimeline in our online documentation. Basically, the demo uses the AnnotationsSource, AnnotationContentTemplate, AnnotationStartPath and AnnotationContentPath properties.
Regarding the installation folder - in case you have installed the latest version of the demos, then yes, you can find the source of the QSF in the installation folder. Please note that the path to the demos in the installation folder depends on the version that you have installed. For example, the path for the Q2 2013 demos should be "RadControls for Silverlight Q2 2013\Demos". That is why, you need not look for the Q2 2011 demos, the demo you need should be in the latest version of the demos - Q1 2013 SP1.