If you're talking about a scanner, put it on NAC Search. It will find the NAC when they transmit. (Who wants to put money down on 293?)
If you're talking about getting a radio programmed so you (presumably you = an entity licensed to operate on said radio system) can use the channel, yes, you need the NAC. Without it, your transmissions will not be heard - it would be the same as trying to transmit on an analog channel without the right CTCSS (or as NC5267 puts them, PL/DPL) tone.
If you are programming a radio, and only want to listen, you can program in F7E (I'm not sure if this works in radios other than Motorolas, but it does work there) to receive "all" NACs. It won't tell you what the right one is, but you'll hear all digital transmissions (so long as they're not encrypted).