A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Here are two possible explanations.
(A) If you are using an Excel VBA add-in which correctly uses the pre-2007 methods for adding a custom tool bar or menu item, and if it is correctly installed, then there will be an "Add-Ins" tab added to the ribbon with a "Menu Commands" group.
In Windows Excel 2013 and 2016, after you click the checkbox in the Add-ins Available list, the Add-Ins tab may not appear until you quit Excel and restart.
Instead, if your add-in creates a 2007-and-later ribbon and does not use the pre-2007 methods, then Excel will not show an "Add-Ins" tab.
(B) Microsoft added some extra security features in July 2016.
Depending on your system and settings and file location, you may have to unblock downloaded files before use.
In your Windows file manager, right-click the XLA or XLAM file, choose Properties > General, check the Unblock box, and click OK.
Then start Excel and use File > Open to load the add-in file.