With OS X 10.7 (Lion) and later it is necessary to disable Gatekeeper before you run PTLens the first time. This must be done for each version of PTLens that you use. For example, if you plan to use the standalone program and a plug-in you must disable Gatekeeper, run both applications, and then you can restore Gatekeeper to its previous setting.
First try opening PTLens so you get an error message indicating the software is from an "unidentified developer". Choose OK to dismiss the dialog then immediately disable Gatekeeper.
To disable Gatekeeper open Finder, choose Applications > System Preferences, and click on Security and Privacy. For OS X 10.9 (Mavericks) or later choose the Open Anyway button after encountering an error indicating the software is from an unidentified developer. For older versions of OS X click on the lock (1) and choose the Anywhere (2) radio button.
Now you may run PTLens once so it is accepted by Gatekeeper. For the standalone program or external editor double-click on the program. For plug-ins open the application using the plug-in, open an image, and invoke PTLens. Once this is done you may restore Gatekeeper's setting.