Small addition: your NL2 itself will never show up in FL, but your MIDI interface will. On newer Nords that have MIDI-over-USB functionality (just having a USB connection is not enough) it would actually appear as that specific keyboard; I think that would be the Nord Stage 2, Nord Piano and Piano 2, Electro 4 and the new Lead 4.
Anyway, a MIDI connection is uni-directional. That is, data only 'flows' from the MIDI output of device A to the MIDI input on device B. So if you want to send MIDI data from your NL2 to FL (via your computer's MIDI interface), you would plug one side of the MIDI cable in the MIDI out port on the NL2 and the other side into the MIDI in port on your MIDI interface. Conversely, if you want to send MIDI data from FL to your NL2, plug a MIDI cable in the MIDI out port on your MIDI interface and the other side into the MIDI in port on your NL2.
On a lot of the simpler MIDI interfaces, the cable will be an integral part of the MIDI interface; if you can't get things to work, try swapping the MIDI cable(s) on the NL2 side. With a 'real' MIDI interface a port might be labeled 'MIDI in', while on a cheaper USB one the cable attached to it might be labeled 'out', because you connect it to the MIDI out port of your device. A bit confusing and non-standard with interfaces that have a cable that is fixed.
Most Nords have a MIDI LED which blinks if MIDI is received by the Nord - I haven't checked if the NL2 has that though. On newer Nords the MIDI LED blinks briefly if it receives a MIDI messages on a channel it is not listening (= it is ignored), while it stays on longer if the messages is actually processed.
If you are using the NL2 as a controller keyboard, you may want to turn 'Local Control' off (check the manual). When you press a key on a Nord, it normally sends out a MIDI message via the MIDI out port
and it sends a copy of that message to the 'sound engine' inside the Nord. If you turn 'Local Control' off, that copy will no longer be sent, so only a MIDI message will be sent via the MIDI out port. If your NL2 is making sound when you're using it as a controller, or if you want it to make sound but hear strange 'echoes', double notes, etc. try turning off 'Local Control'. This 'echoing' of sounds or the double notes mentioned may be caused by the DAW software mirroring (echoing) MIDI messages it receives from the Nord back to the Nord (so the internal 'sound engine' receives every keypress twice, with a very small delay between the two).
I briefly mention MIDI channels: a single MIDI connection consists of 16 different channels, numbered from 1 to 16. You need to make sure that the MIDI channel on your NL2 is the same as what FL is using. Check the manuals for the NL2 and FL on how to set the MIDI channel.
A bit of a brain-dump, but hopefully it will be of some use. Don't hesitate to ask if you can't get it to work, but please try using the search function of the forum first
Edit: oh, and MIDI does not carry audio, it only carries 'meta data' between two MIDI enabled devices. What this means is that when you press a key on your NL2, a message is sent that contains information on what key you pressed (MIDI note number) and how quickly you pressed it (velocity). When you release that key, it sends a message stating what key you released and (in some cases) how quickly you let go of that key. Depending on how you've set up your Nord, it will also carry data about what program to use; not the actually settings, but just the program number.
