This slight modification will allow your hidden sheets to appear on your userform.
wonderful code BTW.
For i = 1 To ActiveWorkbook.Worksheets.Count
        Set CurrentSheet = ActiveWorkbook.Worksheets(i)
         '       Skip empty sheets and hidden sheets
        checkme = CurrentSheet.Visible
        CurrentSheet.Visible = True
        If Application.CountA(CurrentSheet.Cells) <> 0 And _
        CurrentSheet.Visible Then
            SheetCount = SheetCount + 1
            PrintDlg.CheckBoxes.Add 78, TopPos, 150, 16.5
            PrintDlg.CheckBoxes(SheetCount).Text = _
            CurrentSheet.Name
            TopPos = TopPos + 13
        End If
        CurrentSheet.Visible = checkme
    Next i