The guys above hit who the ENG is. The NR guys are referring to qualifying Engineer, which you do toward the end of your JO tour. You go to PNEO for 8 weeks, take an exam, and then interview in DC. If you do not qualify Engineer, you cannot advance in the Submarine force. Very few people fail PNEO, so you don't really have to worry about it until the time comes.
Note that this does not make you the ENG on your boat, it just qualifies you to get slated for the job during SOAC.