Vabbé per completezza posto anche la soluzione a cui sono arrivato per decomprimere(Vb.Net):
codice:
  Dim s As New ZipInputStream(File.OpenRead(percorsoFileDiBackup))

        Dim theEntry As ZipEntry

        Dim mycontrol As Boolean
        mycontrol = True
        '//ciclo fino a quando non trovo una entry
        theEntry = s.GetNextEntry()

        While Not theEntry Is Nothing
            '//nel caso non esista una entry successiva esco dal ciclo
            If (theEntry Is Nothing) Then
                mycontrol = False
                Exit While
            End If

            Console.WriteLine(theEntry.Name)

            Dim directoryName As String = Path.GetDirectoryName(theEntry.Name)
            Dim fileName As String = Path.GetFileName(theEntry.Name)

            ' create directory
            Directory.CreateDirectory(directoryName)

            If fileName <> [String].Empty Then
                Dim streamWriter As FileStream = File.Create(theEntry.Name)

                Dim size As Integer = 2048
                Dim data(2048) As Byte
                While True
                    size = s.Read(data, 0, data.Length)
                    If size > 0 Then
                        streamWriter.Write(data, 0, size)
                    Else
                        Exit While
                    End If
                End While

                streamWriter.Close()
            End If

            theEntry = s.GetNextEntry()

        End While
        s.Close()
Ringrazio ans ed in particolar modo cassano