If the Box for Office plugin is not appearing with the toolbar of an Office program:
Go to the Start Menu or File in the Office program. Click Word/Excel/Powerpoint Options or Options. Click Add-Inns. Select COM Add-Inns 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 make the plugin appear in the toolbar.
If the plugin does not appear after doing this process:
Go to the Add-Inns section again. Select Deleted 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. This should make the toolbar appear in your Office program.