Controlling 23K256 SPI RAM with Arduino
Your problem is likely logic level mismatch. Since your RAM IC is powered with 3.3V, its output HIGH level on the SO pin can't be larger than 3.3V. Your microcontroller however seems to use 5V (looking at the schematics). Now I haven't consulted AVR datasheets, but most CMOS chips recognize a HIGH level when the voltage on their inputs is larger than 0.7Vcc. This threshold is 0.7*5V = 3.5V, so the microcontroller has every right to not recognize 3.3V as HIGH. This may well explain why you're reading all zeros from your RAM.

Note that the maximum input HIGH level your RAM tolerates is Vcc+0.3V (table 1.1) which in your case equals to 3.6V. That's much smaller than the 5V your MCU outputs, so you might have cooked your RAM already. The difference is certainly enough to bias protection diodes into conduction. Some 3.3V ICs have 5V tolerant inputs, this one does not.

Your options are:


Run your MCU at 3.3V. This is often best if your MCU supports that voltage (I think it does) as it avoids the need for level translation completely. Unfortunately this hardly seems possible within the confines of your platform.
Increase Vcc of your RAM (datasheet says it can tolerate up to 3.6V). However, this is going to be unreliable since the threshold is only marginally exceeded, so any voltage variations (and even internal variations between individual chips) may well lead to problems. This also doesn't solve the input level mismatch, so half of the problem remains.
Do logic level translation in both directions. Many solutions to this problem exist, you should search the web (including this website) to get the picture. In short, it's easier to lower the voltage from 5V to 3.3V (a divider is enough, perhaps with a diode) than to do the inverse (you'll need to use a FET or an IC as a buffer). This appnote shows some circuits, as does this page. See also this product from Sparkfun for a similar idea. HEF4050BT is a buffer IC with absolute input thresholds (HIGH is 3V) and can tolerate up to 15V on inputs. I'm sure more possibilities exist, but I personally believe it's best to power your microcontrollers with 3.3V and save the trouble. Many useful ICs aren't 5V compatible (or tolerant) these days.