uses に

{$IFDEF MACOS}
,Posix.Stdlib
{$ENDIF}
{$IFDEF MSWINDOWS}
,Windows
,ShellAPI
{$ENDIF}
;

procedure TfrmMain.actOpenOnlineHelpExecute(Sender: TObject);
const
TMP_URL = 'http://www.rigakukan.com';
begin
{$IFDEF MACOS}
_system(PAnsiChar('open ' + TMP_URL));
{$ENDIF}
{$IFDEF MSWINDOWS}
ShellExecute(0, 'open', PChar(TMP_URL),
'', '', SW_SHOWNORMAL);
{$ENDIF}
end;