Can you please check your Storage Pool if any Virtual Disk group exist or not because without any Virtual Disk Group you can't create read cache disk group.
Steps are straight-forward:
First create a Disk-Group for the pool (you can't READ-CACHE without a back end storage)
Then in the SMU (GUI) select _READ-CACHE and select the SSD(s)
You're done.
I would start with just one SSD per pool (as per the best practices guide) and check your performance.