The POE Adapter Kit has 2 parts- one with a male barrel connector and the other with a female barrel connector.
You will still use your power adapter, although instead of plugging it into the encoder directly you will plug it into the POE Adapter with the female barrel connector and into power near your POE switch.
Next use the end of the encoders Ethernet cable that would plug into the switch and plug it into the same POE adapter with the female barrel.
Next use the Ethernet jack on the same POE Adapter and plug it into your switch.
At your encoder side you will use the opposite adapter with the male barrel plug and insert it in to power receptacle on the encoder and also plug the Ethernet port into the other side of the encoder.
With this setup it is required you inject the correct amount of power into the Ethernet at your switch in order for it to work. Most POE switches cannot power the encoder alone.