The Red Wire

If you insist on ignoring my advice, then go ahead and attach the red wire (or whatever your +5V wire is) to Pin 2 - 5v Power. You could have used pin 4, but keeping power and ground separated a bit helps reduce the odds of accidental contact between them.

CAUTION! DO NOT CONNECT BOTH THE SERIAL ADAPTER’S 5V POWER (RED WIRE) AND THE MICROUSB POWER CABLE IN AT THE SAME TIME! YOU CAN SHORT OUT YOUR SYSTEM AND DAMAGE IT!

Obviously they both have to also be connected to power at the same time for this to happen, but all the same don’t have them both plugged into the board; it’s really easy to lose track of what you’re doing and apply power to both simultaneously.

So here’s the chicken and the egg part: in order to have the /dev/ttyUSB0 (or whatever it shows up as) device available to your serial console software, you’ll need to plug in the USB-TTL adapter. But if you’re also powering the 3B from the adapter, you’ll miss out on the startup info coming out of the serial port; hence my recommendation that you not power from the USB-TTL adapter. Being one to not follow my own advice, I actually do power from the USB-TTL adapter but I also use the Serial program on OS X which can handle the USB device disappearing and reappearing. Did I mention it was expensive? And that there may have been beer involved?