This is probably happening because the folder you are using in Google Drive is not the folder we created for you using our automated process, which is initiated when you follow the Getting Started instructions you should see after creating a new Leanpub book.
When you create a book in our Google Docs writing mode, you need to let us create a new folder for the book in Google Drive for you.
You can't create a folder in Google Drive yourself, and then link it to Leanpub.
We have an automated process for doing this, which is actually easier than you doing it yourself, and less likely to cause errors.
When you create a book in Google Docs mode and follow the Getting Started instructions in order, after you click the blue button you will initially see in Step 3, a link that you can always use to get back to the correct folder in Google Drive will appear in Step 4:
If you are signed in to Google Drive in your browser, when you click that link, you should go to the correct folder for your book in Google Drive.
After clicking the link in Step 4 and going to Google Drive, try starting the Add-on in one of the documents in that folder: you should see that the Leanpub Add-on panel that opens to the right is no longer empty.
(Editor's Note: If you understand this already, and you believe you have done everything correctly, you may have inadvertently started working in a Google Drive folder other than the folder we created for your book. In my personal opinion, the way Google Drive hides important information in order to have a supposedly "clean" design, and the way it tries to hide folder structure to encourage you to search for files and folders, or follow its automated prompts, instead of making it easier to create and follow a coherent folder and file structure for different projects, makes mistakes like this easier to make - it's not your fault!).