Where is Boomerang on Outlook for Mac?

On the "New Outlook for Mac", you may need to customize your toolbar to get Boomerang's buttons to appear.

First you'll need to click the Ellipsis (...) Icon while in the Inbox view, then select Customize Toolbar.

From there, drag and drop Boomerang onto the top row (the toolbar). Then click Done to save the changes.

You'll need to perform the same steps in Compose view as well.