こんな感じで、できると思いますが・・・・
Dim NewFNAME As String, CurFNAME As String
q = Left(ThisWorkbook.Path, InStrRev(ThisWorkbook.Path, "\"))
NewFNAME = ""
CurFNAME = Dir(q & "reserve*.csv")
Do
If CurFNAME > NewFNAME Then
NewFNAME = CurFNAME
End If
CurFNAME = Dir()
Loop Until CurFNAME = ""
If NewFNAME <> "" Then
Workbooks.Open Filename:=q & NewFNAME
End If