On the first use of the add-on, you will need to get to the setup tab and follow the simple two step instructions on how to get your scribd api key.

After that you wont need to worry about the api key anymore as it is stored in your preferences.

That way, you can set it up for your client and all they need to do is to browse for the doccument and publish is on their page.

It is smart enough to adapt the doc size to the width of the containing div too so you don't have to hard code that.