three conditions need to be met for the populate list form array behavior:
1) You must have a parent list
2) You must have a child list
3) You must have the Populate list from array server behavior.
you have already added the Populate list from array behavior, make sure you have the Parent list and the child list set up.
Then select the parent list in design view and click the plus button in the behaviors panel ot access the populate list from array behavior.