Hi,
On the Exchange Admin Ceter (EAC) page, please check whether the status of this APP is enabled or not. If you want your users to be able to use the app, once installed, make sure to edit the app and set it to "Enabled by default".
Apps for Outlook are installed in Exchange Admin Center and from there made available in your Outlook client and Outlook Web Access. So, for further assistance, I would suggest you post in the dedicated forum of Exchange Online, where you can get more experienced responses:
http://community.office365.com/en-us/f/158.aspx
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Regards,
Ethan Hua
TechNet Community Support