Visual Basic,AutoCad

Esto es tema de ActiveX y VBA (Visual Basic for Applications)
Primero debes añadir una referencia a la librería de autocad. menu proyecto referencias ... Autocad 2004 type library..
Dim dwgApp As AutoCAD.AcadApplication
Set dwgApp = New AutoCAD.AcadApplication
dwgApp.Documents.Open "C:\Archivos de programa\AutoCAD 2004\Sample\db_samp.dwg"
' Create the block
Dim blockObj As AutoCAD.AcadBlock
Dim insertionPnt(0 To 2) As Double
insertionPnt(0) = 0#: insertionPnt(1) = 0#: insertionPnt(2) = 0#
Set blockObj = dwgApp.ActiveDocument.Blocks.Add(insertionPnt, "CircleBlock")
' Add a circle to the block
Dim circleObj As AutoCAD.AcadCircle
Dim center(0 To 2) As Double
Dim radius As Double
center(0) = 0: center(1) = 0: center(2) = 0
radius = 1
Set circleObj = blockObj.AddCircle(center, radius)
' Insert the block
Dim blockRefObj As AutoCAD.AcadBlockReference
insertionPnt(0) = 2#: insertionPnt(1) = 2#: insertionPnt(2) = 0
Set blockRefObj = dwgApp.ActiveDocument.ModelSpace.InsertBlock(insertionPnt, "CircleBlock", 1#, 1#, 1#, 0)
DwgApp. ZoomAll
También puedes usar la instancia tardía con CreateObject o con Getobject.
Te sugiero revisarla documentacio que se instala con el Autocad. Allí encontraras ejemplos...