$find() is a general ASP.NET AJAX method for finding controls that uses the ClientID of the corresponding control. More information on it can be found in the ASP.NET AJAX documentation.
In your case I believe the problem is that you are not using the ClientID of the control - if this doesn't help, please open a new support ticket and send us a small sample that shows your current setup. We will check it and get back to you.
the Telerik team