Computer bus frequency and electricity frequency
I think what might be missing here in your understanding is that when we send information on a computer bus, we are sending digital information.  Bits are represented with two states, '1' or '0' - or simply "on" and "off".  A string of bits represents a number in base 2.  We represent these two states (1 and 0) with discrete voltage levels, for the sake of simplicity, something like +5V and 0V respectively (different devices may use different levels to represent their bits but the principle is the same).

So when we talk about a computer bus having a frequency, we're talking about how quickly it can toggle a bit - or a single wire in this case - from a logic '1' to a logic '0' or vice versa as this will determine the maximum bandwidth the bus is capable of (at the bit rate level).  To further divide computer buses into categories, there are parallel buses and serial buses.  A parallel bus breaks out the individual bits of a base 2 number (typically an address or data value) and gives each one a wire.  Thus, the calculation that was quoted holds true for parallel buses.  Then there are serial buses for something like USB like you mentioned.  On a serial bus, all bits are sent on a single wire (two wires in USB's case, but they are a differential pair, so you can think of it as one wire).  The bit rate calculation for that would simply be its maximum "toggling" frequency.

You should not try to relate the above with a sinusoidal AC signal frequency found in AC power. Yes you are correct in that a DC voltage should have a theoretical frequency of 0 - but in the context of digital communication we are not really interested in that.