Welcome aboard, Chevron.
illuminate the player 1+2 and coin add buttons. - How do i power these from a PC PSU molex connecter?
as in do i connect the red and black from teh molex to the 2 connecters of the bulbs?
For 5v LEDs, you are correct.
For 12v LEDs, use yellow and black.
If you're not sure, use 5v. (red and black) -- 12v LEDs will still light up, just not as bright.
Most LED terminals use .250" female quick disconnect crimps.

If you connect power and the LED doesn't light up either swap the red and black wires or unplug the LED and turn it 180 degrees.
NOTE: If you have glass bulbs, they will have a number on them (555, 161, etc.) that you can use to determine the correct voltage.

question for using a arcade button to power on the pc.
do i just connect the ground and normally open connecters to the power switch's 2 pins on the motherboard ?
Do you just want it to power on when you plug it in?
Don't you want to do a shutdown instead of crashing the system by unplugging it?
Are you using a Smart-Strip or similar to power on the monitor?
Does the monitor automatically power on when power is applied, or do you have to push the button to bring it up?
If the monitor does not automatically power on when power is applied, do you want to wire your bartop so that one button brings up both the computer and the monitor?
Scott