Yes, I didn't mean for you to use the microcontroller, I was just showing off. I am fairly terrible at transistor theory, but I'll give you the basics of what you need to do. I'm sure someone who's more knowledgeable will currect me if I'm suggesting something wrong.
Okay, you have 3 wires. 1 wire is ground. When ground is connected to either the shutter or focus wires, they will 'fire' the shutter or do the focusing. You start by connecting the ground wire with the battery ground of your circuit.
To use the transistor: A NPN transistor like a 2n2222 will connect positive to negative through the collector to the emmitter when power is applied to the base. The emmitter will go to ground and the collector will be attached to the shutter wire. You don't want to apply too much current to the base, though, or you will burn out the transistor. Now, I'm not good at figuring out what resistors to use, so I usually start out with a 10k resistor and then go down until the transistor will 'turn on'. Usually 10k works fine for me.
So basicly you will have the ground wire from the camera go to the common ground of your circuit. You have three wires on the transistor, emmitter goes to ground, collector goes to the shutter wire from the camera, and when you apply positive power to the base through a resistor, it will one way connect the two and you'll get your shutter action.
You'll want to download the datasheet of whatever transistor you use.