I created a user in the 42go ISP-Manager but I cannot see him on the system.
Please check that you did not mix up customers and users. A customer can login to the 42go ISP-Manager and manage his sites (and create system users on the tab "User & Email") but he cannot login to the system (e.g. by using FTP, email) because he is not a system user. A customer is an object that exists only in the 42go ISP-Manager, but not on the system.
A system user cannot login to the 42go ISP-Manager, but he can use services such as email etc.