To turn on/off "Design mode" in Excel 2007, follow these steps:

1. Open the Excel workbook that you want to work with.
2. Click on the "Developer" tab in the ribbon. If you don't see the "Developer" tab, you may need to enable it by going to "Excel Options" "Popular" "Show Developer tab in the Ribbon".
3. In the "Controls" group, click on the "Design Mode" button. This will turn on "Design mode" and allow you to edit or add controls to your worksheet.
4. To turn off "Design mode", simply click on the "Design Mode" button again. This will disable "Design mode" and allow you to use the controls that you have added to your worksheet.

Note that "Design mode" is only available if you have added controls to your worksheet using the "Developer" tab. If you haven't added any controls, the "Design mode" button will be disabled.