Hi Roberto,
in 2.0.0 you will find the “Actice Checks” at the new menu structure “Services → HTTP, TCP, Email…”. There you will find “Check TCP port connection”.
With this check you can cack a port from your target system, this means that a check was issued by checkmk server to check port e.g. 3306 on the target system. So, there is nothing to do on the target server to check the needed ports.
Cheers,
Christian