Private Sub Workbook_Open()
 If CheckAddin("ATPVBAEN.XLA") = False Then _
 MsgBox "Please install Analysis ToolPak adding " & _
 "before using macros in this workbook."
End Sub

Function CheckAddin(addname As String) As Boolean
 Dim ad As AddIn, res As Boolean
 res = False
 ' Check for the addin in the Addins collection.
 For Each ad In AddIns
 ' If found, return true.
 If ad.Name = addname Then res = True
 Next
 CheckAddin = res
End Function