Download Context.IO PHP API from https://github.com/contextio/PHP-ContextIO and extract it to sites/all/libraries or sites/sitename.com/libraries. the name of the folder must be PHP-ContextIO (the version number from the end should be removed).
Navigate to the sites/all/libraries/PHP-ContextIO folder from terminal.
Run `composer install` command.
Enable contextio and contextio_admin modules on your site.