ADControl.Restrictions := ADControl.Restrictions + [crNoMove]; 

MyLine := TDiagramLine.Create(Self);
with MyLine do
begin
  Name := 'MyLineName';
  Diagram := atDiagram1;
  //link start point to someblock
  SourceLinkPoint.AnchorLink := SomeBlock.LinkPoints[0]; 
  // link end point to anotherblock
  TargetLinkPoint.AnchorLink := AnotherBlock.LinkPoints[1]; 
end;

MyBlock := TDiagramBlock.Create(Self);
with MyBlock do
begin
  Name := 'MyblockName';
  Left := 10;
  Top := 10;
  Width := 150;
  Height := 50;
  Diagram := atDiagram1;
end;

with atDiagram1.Blocks[0] do
begin
  Shape := bsNoShape;
  Metafile.LoadFromFile('mymetafile.wmf');
end;

atDiagram1.GraphicLib := dglGdiPlus;

procedure TForm1.atDiagram1InsertDControl(Sender: TObject; ADControl: TDiagramControl);
begin
  if UpperCase(ADControl.DControlID) = 'MYBLOCK' then
  begin
    With ADControl As TDiagramBlock do
    begin
      Shape := bsNoShape;
      Metafile.LoadFromFile('C:\MyMetafile.wmf');
    end;
  end else
  if UpperCase(ADControl.DControlID) = 'MYOTHERBLOCK' then
  begin
    With ADControl As TDiagramBlock do
    begin
    Color := clYellow;
    end;
  end;
end;

initialization

RegisterDControl(TDiagramBlock, 'MyBlock', 'This is a caption for my block', 'My category');
RegisterDControl(TDiagramBlock, 'MyOtherBlock', 'This is a caption for my other block', 'My category');

atDiagram1.GraphicLib := dglGdiPlus;