Step 1: To ensure that the add-in is installed and loaded correctly, you may need to restart Teams and Outlook after installing.  Follow these steps:

• Download the Teams desktop app, run it, and sign in at least once.
• After signing into Teams, restart the app.
• Restart Outlook (after restarting Teams).

Step 2: Ensure that the add-in is installed and enabled.  If after all that, you still don't see the add-in, make sure that it isn't disabled for some reason.

• In Outlook, click File and then Options.
• Select the Add-ins tab of Outlook Options dialog box

• Confirm that Microsoft Teams Meeting Add-in for Microsoft Office is listed in the Active Application Add-ins list
• If the Teams Meeting Add-in is listed in the Disabled Application Add-ins list, select COM Add-ins in Manage and click Go…
• Set the checkbox next to Microsoft Teams Meeting Add-in for Microsoft Office
• Click OK on all dialog boxes and restart Outlook.