var oEditer;
function CustomValidate(source, arguments)
{
    var value = oEditer.GetXHTML(true);
    if(value=="")
    {
       arguments.IsValid = false;     
    }
    else 
    { 
        arguments.IsValid = true; 
    } 
}
function FCKeditor_OnComplete( editorInstance )
{ 
    oEditer = editorInstance;
}