Frankfurt Airport

Just look for the signs that have a schematic picture of a train and follow them. Trying to describe how to reach the Fernbahnhof (the one where you would catch a long distance train, not the one served by regional trains) in words will only cause confusion, because you have to go up and down a few flights of stairs and take several turns. Follow the signs and you can't go wrong.

There is good signage to the fernbahnhof (long distance train station). There is a ticket office there where you can buy your tickets to Nuremberg.

I used to think that I knew the Frankfurt airport like the back of my hand, but last month I flew in and they took us to the new concourse A addition (gates 50-69). Nothing was familiar, and I had to follow the signs. Instead of coming into the main hall, which I would have recognized, the signs routed us through the "basement" mall, under the access road, to the Regionalbahnhof, then to the Fernbahnhof. So, my advice is, follow the signs to the Fernbahnhof. It's out across the access road and through the airport center. Lufthansa and most Star Alliance airlines (eg, United) use Terminal 1, and it's a walk through the terminal to the Fernbahnhof. If you come in on another US flag airline, you will probably come into Terminal 2, a different building, and have to take the Skyline people mover from the top floor of Terminal 2 over to Terminal 1. There should be signs for the Skyline, if not the Fernbahnhof. A ticket for the direct ICE to Nürnberg will cost 112€ for 2 people, and the trip will take 2h22m. A cheaper, albeit longer, alternative would be to take the S-Bahn from the Regionalbahnhof to the Frankfurt Hauptbahnhof, then a Regional Express (RE) via Kahl, the first stop in Bavaria, to Würzburg, where you change to another Regional Express to Nürnberg. You can do this using local (RMV) tickets from the airport to Kahl for 7,60€/adult and a Bayern-Ticket (26€) from Kahl to Nürnberg. That way takes 3h52m, an hour and a half longer, but only costs 41,20€ total, a savings of over $100. You don't have to change trains in Kahl, just tickets. Use the German Rail (Bahn) website to find schedules.

If you arrive in Terminal 1 (Star Alliance) you just follow the signs to "Long-Distance Train Station". It's a ca. 10 minutes walk from baggage reclaim. It involves an escalator, in case that's an important information for you.

If you arrive in Terminal 2 (OneWorld) follow signs to exit, curbside, and take the yellow shuttle bus to Terminal 1. It runs every 20 minutes and journey time is under 10 minutes. The bus will drop you off right at the bottom end of bespoke elevator up to the long-distance train station level. Under 5 minutes walk from the bus stop.