You can get a reference to the MasterPage's RadAjaxManager by calling the static method RadAjaxManager.GetCurrent() and passing in the current page. From inside a User Control, this would look like this:
|var ajaxManager = RadAjaxManager.GetCurrent(this.Page);
Additionally, if you have a need to put a RadAjaxManager in your content page or user control when one is already defined in a MasterPage or parent Page, you can use the RadAjaxManagerProxy.
I hope that helps. If you have any further questions, please don't hesitate to ask.