You need to get the taskId from the output of a human task in your previous Interactive activity in the process. Double click the Interactive -> click the activity's "Data Associations" in the "Implementation" tab -> click the "Output" tab -> expand the "execData" node and then expand the "systemAttributes" and scroll down to get to the "taskId" attribute you're looking for. There's more work you'll need to do to get this to work if you're going to use the getPreviousTaskApprover() method but at least this tells you how to get the prevous approver. This blog by Niall Commiskey goes into the detail on the remaining steps you have left to do after this: http://niallcblogs.blogspot.com/2011/06/pinning-tasks-to-user-in-bpm-11g.html.
You might instead want to consider using the getLastPerformer() method. This method can be used inside the human task's assignment tab. To see what I'm talking about, determine the human task that is associated with the Interactive activity you want to assign to the previous participant -> in the BPM Project Navigator tab, expand the business catalog -> expand the "Human Tasks" -> double click the human task that is associated with the Interactive activity -> click the "Assignment" tab -> double click the person icon in the middle of the page next to the text "default...." -> in the dropdown beside the text "Build a list of participants using" scroll up and select "Names and expressions" from the dropdown -> click the plus icon -> click "Add User" -> change the Data Type from "By Name" to "By Expression" -> click the "..." button on the right -> click the dropdown below "Functions" -> click "BPM Functions" -> click "getLastPerformer" -> click "Insert Into Expression" -> click "OK".