You will find that if you have updated to OS X Mavericks (currently BETA 13/05/13) the iOS Simulator will not activate from the Flash Builder IDE. FB will prompt to saying you need to enable assistive devices but Mavericks has removed this option from the Universal Access option in the system preferences.
Here is the work around:
Activate System Events:
System Preferences > Security & Privacy > Privacy > Accessibility > Tick System Evenh
There is no need to restart Flash Builder the simulator should now just work fine.
Hope that helps prevent a lost few minutes on the old google machine.
I hate to dig up an old thread, but this is the top result when Googling for "flash builder assistive devices" and I want to note that this isn't entirely true anymore in the final release.
You need to now:
That should allow you to debug in the iOS Simulator again.
Thanks for pointing me in the right direction.