To add files from your Google Drive please go to the Drive tab. Here you can browse through your entire Drive and add files from there.
You can search for files or browse all your folders in your Google Drive.
To insert a file you already have all you need to to is click on the file and it will be added to your cell.
If you want to share the files you add with your colleagues or team select the correct permissions in the settings tab.
Learn how to upload files directly to your Google Drive.
This is a paid feature, please upgrade so you can use it.