Using "Search" function when assigning Predecessors and Successors.

Good day and Happy New year.  I have a rather simple question, but I've forgotten the process.  I am assigning predecessors and successors in P6 version 16.1.  By using the Relationships tab at the bottom, I click on "Assign" and the  Predecessor and/or Successor box pops up.  All activities are visible.  As soon as I try to "Search", all activities disappear and the green assign button on the right goes gray.  What's the process to use the "Search" feature?