First, to directly answer your question, the USB 5 volts connects to the VUSB and VIN pads. Here's the schematic.
http://www.pjrc.com/teensy/schematic.html
Regarding 6 feet of cable, that's a pretty long distance to send signals. The pins on Teensy and also on a PIC are meant to drive relatively short wires, pretty much on the same circuit board or to close-by parts.
At least 2 types of problems come up when sending signals over long distances.
#1: Ground differences between the 2 locations can cause big problems. The pins on a Teensy or PIC are not meant to handle negative voltages, which can happen if the grounds differ slightly. Teensy 3.1 is 5V tolerant, so it can handle a 3.3V signal that's up to 1.7V too high.
#2: Long wires tend to have signal reflections that can cause stress or damage to the parts, not to mention simply causing unreliable results.
Usually "long" wires are driven by special transceiver chips. These chips give you 3 things. They're able to handle a wider "common mode range", or signals that are slightly below ground or above the power, due to the 2 sides not having exactly the same ground. They typically have some sort of slew rate control, which limits signal reflections and helps prevent radio frequency emissions. They also often have some sort of design for impedance matching (often you supply a resistor) to the cable, which *really* helps give excellent signal quality. Some chips, like the RS-485 ones, transmit the signal on a pair of wires and receive it as a difference, which gives you incredible resilience to noise and interference. Some others, like RS-232, use a wider voltage range.
But adding transceiver chips means more parts, more work, more cost and more complexity. For 6 feet on a non-critical project, you can probably "get away" with just connecting the pins to a cable. I would recommend at least connecting 100 ohm resistors between the pins and the wires. That will give you a little better impedance matching, and if things go horribly wrong (like accidentally shorting wires to stuff), some impedance might limit the current into the chip and lessen the odds of destroying it.