Please open an email in Gmail and then click on "TD Add" - the button at the top, next to Gmail's other navigation buttons.
Also, Ctrl+F5 should force a reload with the Todoist plugin visible.
I did find if i reopened the gmail it showed up. I don't see a TD add
Will todoist link with gmail calender
With Todoist Premium, you can get access to an iCal feed which can be added to any calendar software and sync live with your Todoist's tasks. Google Calendar is one exception, though - it will sync, but once a day.
Please refer to this short video to see how it works: https://www.youtube.com/watch?v=gzGubOy8GuE