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 (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 inside an Ellipsis (...) 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) Icon or the Ellipsis (...) Menu for some accounts, then Boomerang, then the feature you want to use.