The Radio Shack LEDs you linked to are IR LEDs meaning they do not emit any visible light. A good example would be the IR Remote for a TV. You don't see the light.
A typical red LED will generally have a forward voltage of about 1.6 volts with a forward current of about 10 to 20 mA. So let's say 1.6 V @ 15 mA for an example. We have a 5 volt supply.
Vsupply - Vled / Iled so we get 5 Volts - 1.6 volts = 3.4 volts / .015 amp = 226 ohms with the closest common size being 220 ohms so we would place a 220 ohm resistor in series with this LED for 5 volt operation.
Ron