When can you start using the Pirate in Catan Cities and Knights / Seafarers?
From the rules to Sefarers, page 6:


  If you roll a "7" during your turn, you may elect to move the pirate
  INSTEAD of the robber.


From the Cities and Knights rules, page 5:


  The robber cannot be moved until after the barbarians reach the island
  of Catan for the first time.


Since the robber can't be moved until the barbarians attack, neither can the pirate, as there is no robber movement to replace.