Some older games, especially the classics of Atari like Asteroids and Centipede or Midways' PacMan feature flashing start-buttons when enough coins are inserted. It would be a shame to not implement this feature, and fortunately the I-PAC supports it.
Most likely you would use the typical HAPP-startbuttons - see the pictures above how to get a LED inside for this task. This pictures are from http://erikruud.freeservers.com/arcade/buttons.html as I haven't made pics of the ones I made yet.
Unfortunately the I-PAC only drives LED's up to 10mA, regulary you would need something like 20mA. There are low-current LED's out there that consume 5mA or less, but you won't have the possibilities to use blue, white, or high intensity LED's though. If you just want to add the LED's to the controlpanel itself and don't need special colors, you just can get an easy-fitable LED-harness for the I-PAC with no need for additional resistors and circuits.
I wanted to use a blue and a purple (for this you need a so called RGB-LED, which can mix up every color you want), so I made this simple circuit below. This way you could drive enough current for every LED you want. To calculate the needed resistor (R2), take a look at the LED's how to.
The LED-driver I designed. You may use any pnp-transistor you can get - the type mentioned here is out of production, but was the first one where I found a datasheet online. My electronic dealer even gave me a darlington-type instead, but it works anyway! Just be sure you get the right connections with the B,C, E. Ask your dealer about this, there are books out there where all transistors are listed with connections, but these are quite expensive. Maybe you can find the connections online as well.
You'll need three of the above circuits, if you wanna use all possible LED's! Most likely one LED flashes for 1up, the second for 2up, and the third only on some special games.
Take a look at Oscar's Website http://www.oscarcontrols.com - under LED-driverboard you'll find a far easier description how to build this thing! (I'm just to electronic orientated here, I guess ;-) )