The demo uses the Telerik NuGet server for UI for Xamarin. The reason you're having trouble is because you likely have not added the Telerik NuGet server to your NuGet Package Sources. Telerik.UI.For.Xamarin.Trial package is not published to nuget.org, it's published to nuget.telerik.com/nuget.
To quickly fix this, go to this documentation
and follow the steps in the Visual Studio section. In short, you're adding the Telerik package source to the Package Sources so that the Package Manager will be able to find (and install) a NuGet package named "Telerik.UI.For.Xamarin.Trial".
Restore and Build
Once the package source is there, open the the Package Manager you will be able to find the "Telerik.UI.for.Xamarin.Trial" package, Here's a screenshot of the Package Manager, notice that I call out the Package source dropdown, This is important because if you have "nuget.org" selected, the UI for Xamairn Package won't listed).
Using our NuGet server has the major benefitr that it's much easier to update your project to newer versions of UI for Xamarin when updates are released. It also means that you do not need to install UI for Xamarin to every machine that you use for development.
Let me know if you have any trouble. Worst case scenario, say for example you're behind a firewall that doesn't allow the connection, I can update Yana's demo to use local DLLs instead.
Lance | Tech Support Engineer, Sr.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items