Step 22: Build the cube: test the layer
Soldering that close to the body of the LED can damage the electronics inside. We strongly recommend that you test all LEDs before proceeding.
Connect ground to the tab you left sticking out at the upper right corner. Connect a wire to 5V through a resistor. Use any resistor that lights the LED up and doesn't exceed its max mA rating at 5V. 470 Ohm would probably work just fine.
Take the wire and tap it against all 64 anode legs that are sticking up from your template. If a LED doesn't flash when you tap it, that means that something is wrong.
1) Your soldering isn't conducting current.
2) The LED was overheated and is broken.
3) You didn't make a proper connection between the test wire and the led. (try again).
If everything checks out, pull the layer from the cube and start soldering the next one.