FCK editor won't be load

I add this tag:
<%@ Register Assembly="FredCK.FCKeditorV2" Namespace="FredCK.FCKeditorV2" TagPrefix="FCKeditorV2" %>

and drag and drop the control from toolbox to the form Yet I got this:

Server Error in '/SolPortal' Application.
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HTTP Error 404 - Not Found.

what could be the problem?