//TDataModule выбран для примера, реализовать интерфейс можно в любом удобном классе, например главном окне 
 type 
  TDM =  class (TDataModule, IHtmlHelpTester)
  ...
   protected 
     function  CanShowALink( const  ALink, FileName:  String ): Boolean;
     function  CanShowTopic( const  Topic, FileName:  String ): Boolean;
     function  CanShowContext( const  Context: Integer;
                             const  FileName:  String ): Boolean;
     function  GetHelpStrings( const  ALink:  String ): TStringList;
     function  GetHelpPath :  String ;
     function  GetDefaultHelpFile:  String ;
     function  ConvertWinHelp( var  Command: Word;  var  Data: LongInt): Boolean;
  ...

 function  TDM.CanShowALink( const  ALink, FileName:  String ): Boolean;
 begin 
  Result := true;
 end ;

 function  TDM.CanShowContext( const  Context: Integer;
   const  FileName:  String ): Boolean;
 begin 
  Result := true;
 end ;

 function  TDM.CanShowTopic( const  Topic, FileName:  String ): Boolean;
 begin 
  Result := true;
 end ;

 function  TDM.GetHelpStrings( const  ALink:  String ): TStringList;
 begin 
  Result := TStringList.Create;
  Result.Add(': ' + ALink);
 end ;

 function  TDM.GetDefaultHelpFile:  String ;
 begin 
   if  Application.HelpFile <> ''  then 
    Result := Application.HelpFile
   else 
   if  Application.MainForm.HelpFile <> ''  then 
    Result := Application.MainForm.HelpFile
   else 
    Result := ChangeFileExt(Application.ExeName, '.chm');
 end ;

 function  TDM.GetHelpPath:  String ;
 begin 
  Result := ExtractFilePath(Application.ExeName);
 end ;

 function  TDM.ConvertWinHelp( var  Command: Word;  var  Data: Integer): Boolean;
 begin 
   if  (Command =  11 ) {HELP_FINDER}   and  (Data =  0 )  then    //THelpContents 
    Command :=  1    {HH_DISPLAY_TOC} 
   else 
   if  (Command =  11 ) {HELP_FINDER}    and  (Data <>  0 )  then   //THelpTopicSearch; Data=PChar('') 
    Command :=  3   {HH_DISPLAY_SEARCH} 
   else 
   if  (Command =  4 ) {HELP_HELPONHELP}   then   //THelpOnHelp 
    Command :=  3 ;  {HH_DISPLAY_SEARCH} 
  Result := true;
 end ;
...

 procedure  TDM.DataModuleCreate(Sender: TObject);
 begin 
  HtmlHelpViewer.HtmlHelpTester := Self;
...