Hi Stoney,
If the aircraft you are flying is ADF equipped - you may have to open a separate panel window - it's just a matter of dialling in the frequency. You can listen to the Morse ident to verify you are tuned into the correct NDB.
The NDBs are given a set range, just like the real things, by the author of the scenery BGL file. Therefore, if you're a long way from base, you may not get reception. In that case, there might be an enroute NDB which you can utilise.
When you have reception, it's a matter of flying towards the needle of the ADF. As you pass over the station you will see the needle swing 180 degrees.