VBA Solver Reference

I haven't done much testing of this, but you could try it:

Function LoadAddIn(sFullPath As String) As Boolean
    Dim i       As Long

    For i = 1 To ThisWorkbook.VBProject.References.Count
        Debug.Print ThisWorkbook.VBProject.References(i).FullPath
        If ThisWorkbook.VBProject.References(i).FullPath = sFullPath Then
            LoadAddIn = True
            Exit Function
        End If
    Next i
    
    On Error Resume Next
    ThisWorkbook.VBProject.References.AddFromFile sFullPath
    LoadAddIn = Err = 0
    On Error GoTo 0
End Function

The paths are:

Office 2007: C:\Program Files\Microsoft Office\OFFICE12\Library\SOLVER.xlam

Office 2003: C:\Program Files\Microsoft Office\OFFICE11\Library\SOLVER.xla

Office XP: C:\Program Files\Microsoft Office\OFFICE10\Library\SOLVER.xla

You can use Application.Version to tell you what version you're running.