The Extract plug-in is not a Universal application, so it requires a Power Mac PPC (pre-Intel machine) or Rosetta PPC emulation to run, neither of which is available to you You must have an Intel Mac, or you couldn't run Snow Leopard 10.6.x and the latter dropped all support for Rosetta. You need to take advantage of the superb capabilities of CS5 or, even better, CS6.
Sorry.
