Unlike what everybody writes on the internet, going to Plaza Nunoa is quite simple actually, there are at least two ways to get there :
Theoretically, from metro station Las Condes (this station doen_Lt exist!) (on line 1), you should be able to find bus line 104 or 212. But try not to walk on a street other than Providencia or 11 de Septiembre, cause the other streets become quite unsafe at night (I've been attacked on street "Los leones" and lost my credit card and my keys). This will lead you to a bus station near the Club de Jazz de Santiago, at the corner of streets "Jose Pedro Alessandri" and "Avenida Irarrazaval". Take the latter and walk a little to the east.
One other way is to take "el metro", line 4, and stop at station Las Torres. There you can find buses to Plaza Nunoa
Or you can also walk to the Plaza, but it may take quite a long time, and the residential area Nunoa is of little cultural interest, and may be unsafe.
On Friday and Saturday evenings there is many people around. It's a good place for drinking and eating and enjoying some quiet nightlife.
There is also the famous "Club de Jazz de Santiago" where Louis Armstrong played once. Be careful to check the program and watch samples of the bands on youtube before going, you could have bad surprises.