Yo, El Barto! Essentially, compression involves 4 parameters. Ratio, threshold, attack and release. The RNC has these plus a gain knob. The gain knob is a parametric fader, in effect a volume control. The Brits call ratio compression, and threshold slope. Ratio determines the degree of compression. Threshold indicates the level at which compression begins. Lets say your threshold is 100 decibels and your ratio is 3:1. This means the compressor theoretically does nothing until the signal reaches 100 dB. For every 3 dB above 100, the compressor allows 1.
When you get to the point where *nothing* is allowed over the threshold (say a ratio of 25:1 or higher), the compressor becomes a limiter. Attack defines how fast the compressor will act in response to a signal that is at or higher than the threshold, and release determines how soon the compressor will stop compressing when the signal drops below the threshold.
So for your bass, you want to send a pretty hot signal to the compressor with a pretty high ratio. If the signal is not hot enough going in, your volume will vary on softer stuff, as the compressor won't act on the soft stuff at all. You will want the threshold fairly low and the ratio fairly high, so your hot signal does not clip (overload) the preamp.
For your vocals, on the other hand, you want a relatively high threshold and a low ratio, probably not more than 3:1.. Like antilock brakes, most of the time the compressor will do nothing, acting only on that occasional excursion over the threshold.
In the bass scenario, use a slower attack and release, because the compressor will be acting most of the time. With the vocals, a fast attack and release will keep the compressor from acting on stuff you don't want compressed (most of it).
I don't claim to understand the technobabble behind "supernice" but my ears tell me that you turn it on whenever you power up the RNC, and pretty much leave it on all the time. Hope this helps.-Richie