var fckEditor;  
function ValidateRequiredField(sender, args)
{ 
    var value = fckEditor.GetXHTML(true);
    
    if(value == "")
    {   
        args.IsValid = false;
    }
    else
    {
           args.IsValid = true;
    } 
}  
function FCKeditor_OnComplete( editorInstance )
{
    fckEditor = editorInstance;  
}