LEDs are typically 1.7 - 2.2 volts
A plain old LED is usually rated at around 1.7 volts and 10 milliamps and requires a current limiting resistor
If you are providing 5 volts then you need to use Ohms law to figure out the current limiting resistor that you need. Ohms law is Volts = Current * Resistance
For the 1.7 v 10ma LED above, the current limiting resisitor would be:
5 volts total (from power supply) minus the 1.7 the LED needs = 3.3 volts left.
Ohms Law: Volts = Current * Resistance
Resistor = Volts/Current
Resistor = 3.3/.010 (10 miliamps)
Resistor = 330 ohms (a resistor with Orange,Orange,Black stripes)
This being said, there are literally thousands of types of LEDs that have various voltages and currents. For my simulator I used 120+ super bright red LEDs for my home-built custom panel backlighting. The ratings on these were 2.2 volts and 20ma. So I needed to use 190 ohm resistors.
The Phidget 8/8/8 outputs looks like it current-limits them so no resistor or external power is needed. The 0/16/16 needs external power and resistor for each LED.
Hope this helps.
Frank