You've got 2 choices, the first one is probably the easiest:Choice 1:
Power is available in the SATA drive bays where you put in extra hard drives. These are the numbered bays at the top of the case right above where the motherboard is. You'll need an adapter like this:
These might be hard to get into place, take a look at the case and see if you think you'd be able to get the power connector into place, it won't be easy.
On a related note, there is a molex power connector in the drive bay where the Superdrive is located. However, I'm not aware of an easy way to route the cable back into the rest of the case.Choice 2:
There are power connectors on the motherboard at the top left of the board. They are black plastic. Here is a picture of them, the black cables you see in this picture are connected into the board. Its hard to see in the picture but should give you a general idea. Use a flashlight to see them:
This one requires a bit of engineering and 2 cables. You need this one:
and you need this one:
you'll have to cut one of the connectors off of the second cable and attach a 4-pin Molex Female connector onto it. Sorry that there's no easy answer, Apple should really have some Molex connectors in this part of the case, but they don't.