Hello,
We had the same issue to start with.
When you first edit in excel, the add-on should install automatically.
Then, you will need to login to the add-on using your O365 credentials.
If you get the error message you are mentioning, click on the cog in the top right hand corner, and change the server URL to that of your tenant. then press OK.
This should reload with your data.
Julien