First you need to find out on the K-518 display the configuration headers.. (where you need to connect the blank blue / black / white wire (for switching) and the red / black (5 volt power supply) on your k-518 display..

1. You need a 5volt power source cable from the psu to the K-518 display.. Watch out, this 5 volt mus not exceed 5 volt, otherwise you burn out the display.. These displays are very oversensitive for higher voltages.
( there need to be some markings on the back side of this k-518 which indicates Positive + and negative - (the red wire have 5 volt on it and needs to connected with the postive + terminal) the black wire is for the negative connection.. If you connect this right the K-518 display starts running.)

( i cant see the right terminals on picture 4)

2. Then the black, blue and white wires ( you only need two of them..) (when using black and white) the turbo button will set on turbo when the turbo button is not pressed ( in pressed mode the turbo goes off)
When using the blue and black wire (its the other way around) Then turbo will working when pressed the turbo button,if the button is de-pressed the turbo function is off

The connector with `turbo SW` written on it needs to connected with the turbo SW on your motherboard.. The blank one needs to connected with the K-518 display (you need to look for the right connection)

( the bleu, black, white wires are indeed not for power the display, but this is for switching the display and turbo led in these two modes (non-turbo and turbo mode)

You need a seperate black / red wire that mostly are on those molex splitter cables (or just use the 5v red and black negative from your power supply.

I have the same LCD display in one of my cases. Here's pinout of first row:
|T+| |T-| |L| | C| |H| |5V| |G|

T+ and T- are power for turbo diode.
Low, Common, High are signal coming from motherboard thru turbo button to LCD. When you connect High and Common on motherboard, the LCD will show f.e 25 MHz with turbo depressed and 100 MHz with turbo pressed. Connected Low and Common works vice versa.
5Volt and Ground are power for LCD.

For configuring numbers on display, you will need to experiment, because there isn't on the web accurate map of K-518 which would tell what segment will lit up depending on if turbo is pressed or it's not. Attached picture should give you an idea on how to solve this puzzle 😀

Welp, plugged all the wires just like Robin4 said — there actually was a double wire going from the PSU via a Molex connector. It worked perfectly at first: it showed 133 (my CPU actual frequency) with Turbo button released and did not show anything with Turbo button pressed.

After a few minutes some segments stopped working or began blinking. After a few minutes more, the LED stopped working altogether.

Gonna try and reshuffle the wires/jumpers this weekend.

So, I've managed to get this all to work. Turns out, I mistook the connectors and used t+/t- instead of the 5V ones, which fried the LED. Found an S-305H on a spare case which fit. Used the schematics on the site and it now shows the proper frquency.

The problem now is that the Turbo button doesn't really do anything. According to NSSI, my CPU benchmark stays the same regardless of the button.