use the mouse wheel to stop the vertical flashing squares on the left. Then use the mouse wheel to cycle through different patterns. both strips of squares (horizontal and vertical) are divided into 4 sections of 4x4 squares, so when you use the mouse wheel it will change just that 4x4 section. The aim is to get the bright green squares matching the black squares on the horizontal strip. The top 4x4 section on the vertical strip matches the left most 4x4 section on the horizontal strip. Once they all match, press the 'denied' message, and you'll get a completion and hacking icon. click on the icon, then it appears in your inventory. use the new inventory item on the thing you want to hack.