At the moment our UI for Xamarin.iOS components do not support design-time, hence even when you try to browse the Telerik.Xamarin.iOS.dll manually, you do not get the components in the toolbox. Please vote here
in order to increase the priority of this task in our backlog.
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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