I've done the "LED thing" a few times...
First, you have to identify the forward voltage. In this case, it's 3.4, with a max of 4.0.
Then you have to size your supply, and this bit is going to be a bit tricky here: Ni-Cads usually drop off from 1.5 to 1.2 pretty quickly, then drop gradually to 0. I cna't remember how Ni-mHs behave, so you might have to do a little digging.
They are however, AAA batteries, so I'll work with the assumption that they provide 1.5 volts; you can figure out the details on your own.
The LEDs are also rated for 20 mA typical (30 max) current.
So the first thing is to figure out what voltage is left for the resistor. Assuming that you'll be shooting for a typical setup (you can run them a llittle higher, for a bit more luminosity, as long as you don't exceed the specs), you'd have 3 * 1.5, or 4.5 volts as a supply. Subtract 3.4 for the LED, and you have 1.1 volts going through the resistor.
Current stays the same, at 20 mA (0.02 amps). Using V=R * I, where V is voltage, I is current, you can then calculate R: 55 Ohms. Of course you won't find a 55 Ohm resistor, so you pick the next highest value available (56 Ohm, I believe), so that you don't let more current through, than you intend.
That's it. Good luck!
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