I used 3.0v LED's. They come in different sizes, but as Kevin mentioned, the small ones can go almost anywhere. I bought just the bulbs and added my own resistors, 1000 ohms, 1/4w. I believe the lighting outputs on the LM3-s output the full track voltage, which is required to supply older incandescent bulbs, but I'm not positive about that. Order the correct resistor size for the bulb's current rating. This is what Ring recommends:
Use 1000 ohm 1/4w resistor for 15ma or 750 ohm 1/2w for 20ma.
I hope this helps, but if you are still confused, don't worry. We've all been there.