Even after the MAC address is added in cnMaestro the APs still need to know the hostname of your cnMaestro server in order to contact it, which is why it didn't work for you.
This article describes all onboarding options, including zero-touch onboarding using DHCP server options. If you need further details on setting up your particular DHCP server we have articles in the knowledgebase describing the steps for various popular DHCP servers.