Ok, here one way to invoke the Solver addin.
VB Code:
  1. Dim oCBmnuTools As Office.CommandBarPopup

  2.     Dim oCBmnuSolver As Office.CommandBarButton

  3.     Set oCBmnuTools = Application.CommandBars("Worksheet Menu Bar").Controls("&Tools")

  4.     Set oCBmnuSolver = oCBmnuTools.Controls("Sol&ver...")

  5.     oCBmnuSolver.Execute