It seems your browser doesn't support fullscreen API. The extension shows fullscreen button only in browser's which are supported ability to show content in full screen. For example latest versions of Google Chrome, FireFox and Opera are supported this feature as well as IE 11, but Safari on iOS doesn't support it. Fullscreen button is also not shown if a book is loaded into <IFRAME> (for example when a book is shown into a lightbox).
Set "Output image format" parameter to "PNG" and increase value which is used for "Quality" parameters to increase quality.