The tutorial makes reference to the TL0xx (see step 3). Those opamps do not have GND pin but Vcc-.http://www.ti.com/lit/ds/slos080j/slos080j.pdf
Thus, I believe the GND in the OP-AMP symbol of Amanda's schematics should be Vcc-.
Also, there are two 9V batteries connected in series to power the OP-AMP.
- The positive of the top battery supplies +9V and is your +Vcc for the opamp.
- The negative of the bottom battery has -9V and is your -Vcc for the opamp.
- The negative of the top battery (or the positive of the bottom battery) is your OV reference (float ground) for the Arduino.
I hope this helps. Regards!