To administratively disable (shut down) or enable a port, first go into edit interfaces mode, then:

To disable:

set interfaces <interface> disable

To enable:

delete interfaces <interface> disable

To look up MAC addresses on a Juniper switch (running JunOS), log on to the CLI, then type:

show ethernet-switching table brief

You can narrow down the search as well:

show ethernet-switching table brief | match 00:11:22:33:44:55