The one in Jay's picture is tuned to 258kHz. The selector switch on the right selects which range of frequencies is tuned and you rotate the dial referring to the appropriate scale to a position that equates to the frequency you want. All NDBs are low frequency around the bottom of the medium wave broadcast band so their frequencies are in a few hundreds of kHz.
Just to make it a bit more fun, the scale might be inaccurate by a few kHz, which is why you need the Morse identifier to confirm you are tuned to the right beacon.
Most NDBs are amplitude modulated with a tone so you don't need a bfo. Bfo would be needed to hear the tone (so you can read the Morse) if it is carrier wave only (A1 modulation: that means the Morse is sent by turning the signal itself, or 'carrier wave' on and off.
1940's technology but good stuff nonetheless.
Last edited by Heston; 11th Jan 2017 at 11:08.
Reason: Added a bit