Hi there,
I've got the MARS111 mech and a credit board, and have it hooked up as you require.
I'm working from memory here abit, so bare with me!

You need to supply the credit board with 12V and 0V and this in turn powers the mech. via the ribbon cable as you said. You don't need to worry about what the mech. outputs are, only concern yourself with the credit board.
Don't bother with the counter/meter outputs, unless you want to buy an electronic counter to count the usage of the machine.
You need to connect 'coin 1' on the i-pac to the 'credit pulse output' and ground (0v).
Any you will likely have an output called 'lamp' too, which will power the coin lamp when connected to 0v or 12v as well. My credit board actually flashes the lamp when I'm at 1/2 credit!! A great little feature.
You can then set the price of play using the dip-switches. Here's an example of how the switches can be set:
DIP SWITCH
Price of play. 1 2 3 4 5
5 0 0 0 0 0
10 1 0 0 0 0
15 0 1 0 0 0
20 1 1 0 0 0
25 0 0 1 0 0
30 1 0 1 0 0
35 0 1 1 0 0
There is also likely to be a second dip-switch on your board which set's things other than the price of play. Namely the length of the credit pulse sent to the i-pac (fast/slow), and 'bonus credits' for using coins of higher denomination. i.e. Did you ever notice that occasionally a machine set at 20p a go would give you 6 credits for entering a