The skin you have downloaded could be use es an external resource only. The skins you could see in the skins property are the those that are embedded into the project distributed with the Telerik`s DLL.
All other custom skin are external resources and could not be included into the DLL.
How to load skins from external assemblies
explains how you could create your own DLL with custom skins, which will be used as embedded and not external resources.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed