According to the FB terms and conditions (and, I would assume, NW), there is NO access to DL or CO lounges when flying within North America (US, Canada, Mexico and the Carribean) if you're not a WorldClub member. However, some agents don't always know or apply the rules correctly so you may succeed in Newark. If you were FB Plat, you would have access in Detroit (access is now allowed on ALL NW intineraries, including domestic) but since you're NW plat, I think you may be out of luck. Interesting that, in this case, it seems that it's better to be an elite with another program instead of a "home" elite since besides lounge access FB plats also qualify for complimentary upgrades on NW as well as pre-boarding, elite check-in, etc.