A simple answer to the missing "Add" button in GMail
This is actually an answer, not a question. There were too many questions about not seeing the "Add" button in GMail, after installing the extension (GMail, not Chrome, etc). I didn't see this answer in the questions, so I thought I'd share.
If you have the Preview extension enabled in GMail, you probably will not see the Todoist Add button. However, if you look for the icon at top right of the email message you're previewing that looks like an arrow up and to the right, click that. It pops the current email message out into its own browser window.
You'll now see the Todoist Add button, which functions very nicely, thank you. Of course, it's an extra step to have to open and close the window each time, but it's less hassle than turning the Preview extension on and off.