I guess first question is does FCpX successfully launch other apps? If so, then does yor script work when launched on its own? If so, then I have no idea. Maybe check that your script is saved as an app rather than a script?
I wrote something similar a while back although I haven't used on a while. I used Automator and the notification centre to show the alert and Growl/Prowl to display the alert on my iPhone.
Probably not much help but unless some Yosemite update has changed the way your script execute, it should work for you.