The current RadRotator's version doesn't allow to set RadRotator's initialItemIndex property on the client. The only one way to achieve your goal is to set InitialItemIndex on the server via AJAX.
You can find a simple source code which illustrates the fore-mentioned technique in the following KB article:
Let us know if you have any other questions.
the Telerik team
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? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.