Check out Figure 10 in this document on how to wire up a photodiode with a resistor:
You won't even have to apply a reverse voltage to the photodiode unless you want/need it to react faster to light changes. But since in your case the limiting factor will probably be the analogRead or the upcoming multiplexing you should be fine without it.
If that doesn't help then show us a complete diagram on what and how you wired things up.
Good luck!
Tom