The CyberSEO plugin version 6 has been released in 2012 and its development was stopped in 2015. In 2016 CyberSEO 6 was finally replaced by the newest and most advanced CyberSEO 7 “Revolution”.
Now the obsolete version 6 is freeware and you can download it from this page for free. The plugin is not encoded and doesn’t use source code obfuscators like IonCube etc. So you should just extract the contents of the ZIP archive into your wp-content/plugins/ folder and activate the plugin.