Hello Emrah Yigit,
There is no documentation present at the moment regarding RadScrollViewer and its functionality, but I will try to shed some light on how it works. Actually RadScrollViewer is designed to work only with RadElements so it can contain and scroll only descendants of RadElement. It does not accept addition of any controls to it.
If you require adding controls you should use the RadPanel control instead of it. RadPanel's behavior is similar to the standard Panel control but RadPanel supports some great features of TPF like transparency, gradients, etc.
I hope this helps. If you need further assistance please write us back.