The new skins are in a seperate assembly. Here is some more information on the new skins assembly:
Separate Skins Assembly
Having been thinking on how to optimize the size of the Telerik ASP.NET AJAX assembly, we created a separate assembly, Telerik.Web.UI.Skins.dll, which will contain all ASP.NET AJAX skins except for the default one starting Q3 2011. In order to facilitate the adoption of this feature, the Q2'11 Telerik.Web.UI.Skins assembly holds only the four newly introduced skins.
See this blog entry: New Telerik AJAX Skins Assembly
You will not need to register this additional dll explicitly on your pages but simply copy and reference it either from the /bin folder of your project (see the screenshot below) or the Global Assembly Cache (GAC), which ensures seamless backward compatibility with previous versions of the Telerik AJAX controls. Thus the Telerik.Web.UI.Skins assembly will be automatically referenced/loaded by triggering mechanism initiated when one of the newly introduced skins mentioned above is specified for a Telerik AJAX controls, either from design-time or at runtime. Note that in case you do not intend to apply Transparent or Office2010 type of skin, you do not need to copy and reference the Telerik.Web.UI.Skins assembly at all and solely the Telerik.Web.UI.dll will be loaded in the memory.
Hope that helps,