Link tracking in Mail Merge helps you analyze how subscribers interact with your email campaigns. When you enable click tracking in a Mail Merge campaign, the addon tracks how many email subscribers have clicked links in your email messages and the links that were clicked.
Mail Merge for Gmail lets you easily include different file attachments for each email recipient. The add-on is integrated with Google Drive and you can use the built-in File Picker to attach one or more files from Drive into your emails.
If your Google Add-ons are not working as expected or if you are unable to install Google Docs add-ons from the Chrome store, it is likely that your Google Apps admin has disabled the setting that allows domain users to use add-ons.