Fatal error: Class 'NextendFilesystem' not found

This error happens, if you don't have the Nextend plugin installed or activated. If you don't have it installed, then open up your Accordion Menu's installer (accordionmenu-wordpress-x.x.x-pro.zip), if you don't have it download it from our website, then in the nextend-accordion-menu/nextend/ folder you will find a nextend.zip file. You should install that at your Plugins -> Add new -> Upload

Or unzip it, and upload the nextend folder into your wp-content/plugins/ folder, and then you will be able to activate it from your backend!