You can Install Yoxel Inbox app from the Office365 store using the following link: Yoxel Mailbox Assistant

You can also install the add-in while you're in your Outlook:

1) Click the "Store" button in the Outlook ribbon:

2) Type 'yoxel' in the search box and you should see the add-in. Activate it.

4) Once the add-on is installed select an email to read in your Inbox and you should see the Yoxel add-in icon. 
Click the icon to open the task pane (side bar) and follow the Authorization prompts to activate the add-in.