This is right from the Cognos Knowledgebase:
(Doc KB905639CS )
Sub Main
Dim objimp As Object
Dim objrep As Object
Set objimp = CreateObject("Impromptu.Application"

objimp.OpenCatalog "C:\Test\aaa.cat"
Set ImpRep = ObjImp.OpenReport("C:\Test\aaa.imr"

objimp.Visible True
AppActivate "Impromptu - [aaa.imr]"
SendKeys "%F",1
SendKeys "A",1
SendKeys "{TAB}",1
SendKeys "{DOWN 8}",1
SendKeys "{ENTER}",1
SendKeys "+{TAB}",1
SendKeys "D:\test.xls",1
SendKeys "{ENTER}",1
Set objrep = Nothing
Set objimp = Nothing
End Sub
Hope this helps.
Regards,
Dave Griffin