Your link is not opening for me (times out, my problem I think).
Post your op amp circuit. A simple non-inverting summing amplifier with a gain of 50 running from +5V & Gnd with 2.5V of DC offset added should let you see a ~0 to 5V signal coming out for the arduino to convert.
http://masteringelectronicsdesign.com/how-to-derive-the-summing-amplifier-transfer-function/