The encoder is attached to the motor i referenced in the link
It is the same motor/encoder used in the instructables example here
The tech page for the encoder on the motor is here
http://repository.sainsmart.com/index.php?share/file&user=bbu_sainsmart&sid=KgPpwOBr
The encoder is attached as in the attached picture
Motor+ and Motor- go to M1 on the adafruit shield
