If it is an ATX supply, just tie the green and one black together. If it still doesn't turn on, get a 5 ohm, 5 watt resistor and place it between one of the red leads and a black one. If it STILL doesn't turn on, it may be defective.
BTW, For our European friends, I have a 350 watt power supply modified for foam use that I will sell for $20.00. It is rated for 16 amps on the 12v section. I have broken out the 5v and 12v sections on heavier wire and paralleled the power wires so that you can also run a separate wire heat transformer with one l power cord. Shipping is whatever it costs. It is a
220v input only.
I also have a 240 watt power supply modified for foam cutting. It is dual voltage and the 12v is rated for 7 amps. It also has a remote power switch lead. This one is $10.00 plus shipping.
Anyone who wants a modified power supply can contact me and I will give a price for your specs.