SSH to VirtualBox Mac Host Linux Guest using NAT
After adding a port-forwarding rule (e.g. SSH TCP 10.0.0.10 2222 10.0.2.15 22) enter in the host's Terminal.app:

 ssh user_name@host-ip -p forwarded_port


Example with the rule above:

 ssh user_name@10.0.0.10 -p 2222


Don't use identical ports in the rule (host's port ≠ guest's port) like in your screenshot. user_name has to be the name of a remote access enabled account on the guest OS.

You may even skip the IP addresses (10.0.0.10 and 10.0.2.15) in the rule and either use:

 ssh user_name@10.0.0.10 -p 2222


or

 ssh user_name@127.0.0.1 -p 2222


or even

 ssh user_name@localhost -p 2222