For SharePoint, I think it is best to use the "standard" ASP.NET ScriptManager control. If I recall correctly, there are some SharePoint security issues with the RadScriptManager that have to be configured before it will work correctly.
That said, the regular ScriptManager should work with RadTabStrip. If the control is rendering and the tabs are failing, it's usually due to a JS error on the page. Can you look at your JS error console and let us know what errors you're seeing?