hmmm...select the root joint and the last joint wich will define spline IK system.
Then select the curve in outliner and go to component mode(F8).
Select any CV and move it around it should affect joints rotation!
Clusters are not crutial for spline IK, they are here only to easier select CVs without going to component mode.
I would suggest you to open HyperShade or Hypergraf window and studie node structure of every IK system!