Is there a way to get the review tab to show up in Excel web app with SharePoint 2013 on-prem?  Users want to be able to use the "add comments" feature while in the excel web app.

Hi,

That functionality isn't available in Excel Web App on-premise. I just applied the Oct 2014 CU to WAC and do not have it, so the feature just hasn't made it down yet.

You can use the "add comments" tab in excel client. 

Here is a similar post, you can take a look at:

https://social.technet.microsoft.com/Forums/office/en-US/b0f3ab85-8f46-484b-8fdb-407f57846b61/office-wep-app-review-tab-is-not-disponible?forum=sharepointgeneral

Best Regards,

Lisa Chen