If the Box for Office plugin is not appearing in the File Menu of an Office program, try these steps:
Navigate to the File Menu in the Office program. Click Word/Excel/Powerpoint Options and click Add-Ins. Select COM Add-Ins from the Managed drop down menu and click Go, as seen in the screenshot below:
Select the Box for Office plugin and click OK as seen here:
This should enable the plugin appear as expected.
If the plugin does not appear after doing this process, please follow these steps:
Go to the Add-Ins section again. Select Disabled Items from the Manage drop down menu and click Go as seen in the screenshot below:
Select the Box for Office plugin and click Enable. Then restart the Office program. You should now see a Box option in your File Menu.