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:
- Try to run iOS Simulator debug from Flash Builder. Select "Yes" in the menu that it gives you
- Open System Preferences > Security & Privacy > Privacy > Accessibility
- Make sure System Events.app is selected
- Restart Flash Builder
- Back in System Preferences, an option for Flash Builder.app will now be present as well. Make sure this is selected too
That should allow you to debug in the iOS Simulator again.
Thanks for pointing me in the right direction.