It does not say, but winkeyless uses the same LEDs, so maybe you could shoot him an email http://winkeyless.kr/product/1-8mm-led/
Nah, a 5V power supply, a multimeter and a decade resistance box are all I need to discover what I want. 
Or just a multimeter that can measure diode voltage. You can pretty much assume ultrabright LEDs will have forward voltage drop of 3.5V, except for reds which might be as low as 2V.
If you have a keyboard that powers all LEDs in parallel (i.e. one resistor per LED), start with the max current you want to supply all LEDs.
Let's be conservative and choose 400mA, leaving 100mA for powering the microcontroller.
Divide that by the number of LEDs. Let's say 61 keys for a standard 60%... so 6.55mA per LED.
If you were using ~3.5V forward voltage drop LEDs (e.g. white, blue, green, etc.) then the resistor value would be: R=V/I = (5V-3.5V)/0.00655A = 229 ohm
However, if you wanted to be compatible with LEDs with forward voltage drops as low as 2V, then do: R=V/I = (5V-2V)/0.00655A = 458 ohm
Therefore, something like 470 ohm is a good choice for a 60% keyboard, and be compatible with white or red LEDs.
Reversing the math, white LEDs would receive 3.1mA and red LEDs would receive 6.3mA.
Now, if you were ONLY going to use LEDs with forward voltage drop of 3.5V (i.e. never use red LEDs), then you can use something closer to 229 ohm, so the LEDs get closer to 6mA each.