Hopefully we can get you up and running quickly, but first a question:
Are you running the SPARK MAX in Brushless (NEO/NEO 550) or Brushed Mode?
The SPARK MAX Data Port matches the same pinout as the SRX, so the encoder should work with the default ribbon connection in Brushed Mode only.
In Brushless Mode, the default encoder pins are shared with the NEO hall sensor lines so you cannot use an external encoder in the default configuration. This is the purpose of Alternate Encoder Mode. There are instructions on which pins and how to connect to them on the page I linked to.