so there are 3 wires on that switch:
Notice that the three terminals on the switch are labeled NC, NO, and COM. That's Normally Closed, Normally Open, and Common.
When the switch is at rest (not under pressure, the "normal" state) the COM terminal is connected to the NC terminal. We don't care about this condition, so ignore the NC terminal altogether. Nothing goes to it.
When the switch is under pressure, COM is connected to NO. That's the contact we care about.
Now, if you chose to wire this direct (no relay) then connect a line from the battery, through a fuse, to the NO terminal. Then a line from the COM terminal to the pump red wire. Then ground the pump black wire. Know that in this configuration, when the switch fails from the load of the pump welding the contacts together, the pump will run endlessly until you pull out the fuse.
If you chose to run the relay (which you should) then do the following:
From a fused, switched +12 source (such as the feed to the radio) run a wire to terminal 86 of the relay (the coil + terminal). Run a wire from relay terminal 85 (coil -) to the NO terminal on the switch. Run a wire from the COM terminal of the switch to ground. Now, run a line from the battery, through a fuse, and to terminal 30 of the relay (the COM contact of the relay's internal switch). Last, run ye olde wire from relay terminal 87 (the NO contact of the relay's internal switch) to the red wire on the pump, and ground the pump's black wire.
Note that you can pick up a common 20A automotive relay at any auto parts store.
Note that I have deliberately reversed the polarity of the switch (COM vs. NO) relative to the Devils' Own diagram. If you wire it their way, the switches' unprotected NC terminal will have +12 on it whenever the car is not in boost. I consider this a slightly dangerous condition.