I have the Office Insider release of Mac Excel 2016/Office 365. It is more recent than your version, but the middle icon is still grayed out. On Windows Excel VBA, that icon returns you to the worksheet. In Mac Excel 2016 VBA, I click the yellow circle at the top left corner of the VBA window to minimize it and accomplish the same thing. Alternatively, if you leave a small portion of the worksheet window exposed, you can click on that to jump back to the worksheet.