If you are using the Excel Online web app there is no Developer tab available. The Developer tab is included only in the desktop software. In Mac Excel the Developer tab can be activated in either of 2 ways:
- Excel> Preferences> View — Put a check in the Developer tab checkbox at the bottom of the pane, or
- Excel> Preferences> Ribbon & Toolbar — On the Ribbon page scroll in the right column & check the Developer checkbox.
Availability also can depend on the Version Number & type of License as indicated in Excel> About Excel.