After the successful installation of Mailtrack add-on, please, remember to re-start your Gmail app. Close the Gmail app completely and open it again. If you don't know how to close completely your Gmail app, shut down your phone and switch it on again.
If installation using your smartphone is not successful, please, try to install it again using your desktop computer or laptop. Even if you install it using your computer, later it will appear in your Android smartphone automatically.
If the installation has been successful you will see the button "installed" in the Mailtrack for mobile page. However if you still can't see Mailtrack in your Gmail app, please, verify that you installed Mailtrack for mobile for the right email account. In the upper-right corner of Mailtrack for mobile page you will find the email account where it is installed, if not the right one, change the account and install it again.
Be in mind that Mailtrack for mobile is not an app but an add-on for Gmail app. This means that Mailtrack for mobile will not appear as a separate app in your phone home screen, but only inside the Gmail app at the very bottom of each email conversation.
Emails send normally from your Gmail app will not be tracked, you need to send them from the Mailtrack add-on. Mailtrack add-on is located at the very end of an existing email conversation. You can "send tracked reply" or "compose (new) tracked email".