You can (and should) use Composer to install the client.
This is done by adding the following line to the require section of your composer.json file
"onetwo360/onetwo360-api-client": "dev-master"
Now all you need to do is run the installation like this
$ php composer.phar update
To test the client simply cd to the root dir of your project and use the built-in web server in PHP.
$ php -S localhost:8080 -t vendor/onetwo360/onetwo360-api-client/examples/
You can now point your browser to http://localhost:8080/ and test it out.
Composer generates an autoloader file that you need to require in order to use the library.
require 'vendor/autoload.php';
You should now have autoloading for all the libraries installed through Composer.