Hi,
You will need a SET chip to activate the bill validator.
You pop out your game chip and put in a SET chip. Do the config, pop out the SET and put your game chip back.
See Ricks FAQs on the NLG home page for detailed instructions for the SET chip.
Basically the S+ does not know its denomination. When you drop in your coin - its compared to the coin in the coin-comparator and then gives a credit to the machine.
The coin just has to match and it doesn't know if its a penny, a dime, a token or a Peruvian Solae.
The Set chip tells the bill validator that the machine is .25 and that then tells the validator to give 4 credits for a dollar.
Some people have quarter machines but SET the validator to .01 so they can quickly add credits to the machine. Ie 100 credits for a dollar, 2000 for a 20..
If you are picking up a SET chip you might also want to get a CLEAR chip, which brings the machine back to factory settings.
Most of the vendors on our main page can set you up with both.