I’m using Outlook on the Web and can’t find Boomerang!
Outlook on the Web recently updated how it looks, so if you're on the "New Outlook" it is a little tricker to find the Boomerang functions.
When composing a new message, down near the bottom should be an add-ins icon button (if you hover over it, it is labeled as "More Apps"), to the left of the last saved timestamp. Click on that icon, then Boomerang, then select the feature you want to use. For some accounts, the Boomerang button is hidden inside a 3 dot menu rather than a More Apps button.
From the Inbox, you can activate Boomerang features by opening a message, then clicking the add-ins/more apps button (or the 3 dot button), then Boomerang, then the feature you want to use.
To get the Boomerang icons to permanently be in the message options (instead of buried), go to the Settings Cog/Wheel in the top right, then "View All Outlook Settings" (bottom of the pane), the "Customize Actions", and enable Boomerang in the "Message surface" section as well as the "Toolbar" section. Hit save (top right) before closing the settings. We have a video online that shows the steps.