Text boxes aren't supported in Excel for the web. You can't add or view them in the worksheet there.
If you have the Excel desktop application, here's how you can work with text box controls there:
Click Open in Excel and follow the steps to add a label or text box to a worksheet.
But remember that these text boxes won't be shown when you save your changes and reopen the workbook in Excel for the web.
If you don't have the Excel desktop application, you can give it a try. Or, you can buy it here, when you're ready to switch.
