Problem
You cannot see the Add Ins tab in Excel, although you have Add Ins installed.
Solution
Disabling and re-enabling all Add Ins installed in Excel resolves this issue.
Steps:
1.) Open Excel
2.) Click on File (top left corner)
3.) Select Options in the File menu
4.) In the Options dialog, select Add-Ins and on the bottom of the dialog, click the Go button next to Manage: Excel Add-ins
5.) On the Add-Ins window, uncheck all Add-Ins and click OK.
6.) Follow steps 2.) to 4.) again to get back to the Add-Ins window, and check all the Add-Ins again. Click OK and you should see the Add-Ins tab in Excel.