I am a new Maxthon user (at least on Mac - tried a previous version long ago on Windows). I just installed it through the app store this morning on my Macbook.
It seems the extension center for Mac http://www.maxthon.com/mac/ext/ isbehind.
Extensions on this site can only be used by the user of Maxthon Classic Version (V4.1.3.5000), please download the browser of this version if needed!