The hardest part is usually getting the numbers you need, especially when buying from "black box" sources like eBay, etc. Once you have the numbers, the math is pretty simple:
R = (Vs - Vf) / If
Vs is your supply voltage (e.g. 5V).
Vf is the rated forward voltage of your LED at your desired current. This is typically given in the datasheet for the "ideal" operating current. For red and green LEDs, it's about 1.7V. For blue and white LEDs, it's usually closer to 2.2V.
If is your desired operating forward current. Again, this is typically specified in the datasheet. They'll give both a "typical" or "preferred" value and an "absolute maximum". Use the former. Vf (see above) is typically specified at this "typical" operating current. For normal LEDs, go with 10-15mA. For "super bright" ones, it's usually closer to 40-50mA.
To make the units work out, you need units of volts and amps. Your probably have milliamps (mA). One amp is 1000mA, so to convert mA to A, divide by 1000 e.g. 15mA is 0.015A.
You'll probably end up with a value you can't actually buy. Round up to the next standard value.
EXAMPLE:
I have a red LED spec'd at 1.75V at 15mA. I want to run this off of a 5V nominal supply.
R = (5 - 1.75) / (0.015) = 216.67 ohms. Round up to 220 ohms, the next standard 5% value. These are readily available at e.g. Radio Shack, etc.
Anyway, I suspect that the 5V supply to the controller can readily handle an extra LED or two. I wouldn't use a ton of superbright ones, though.