It is a known issue with recent versions of MacOS. It is very picky which applications can be run.
You can try one of the following solutions:
(Method 4 usually works for most people)
1. If you unzip the file on a Windows machine and transfer the file to your Mac with a USB stick it should work.
(I have also heard that it works if you download the zip using Chrome on MacOS and extract with Keka)
2. MacOS is picky about which programs can be run (the first time) by double-clicking. To run a program that says "this can't be opened", right-click the icon and choose "Open". You'll be given an option to open the program, and from then on it won't bug you.
3. For Mac's having trouble installing FPV Freerider try going to the Mac's System Preferences>Security and Privacy> General Tab and check the box "Anywhere" to see if this allows your Mac to install FPV Freerider.
4. For anybody who is still having the issue of the simulator not opening you need to open up terminal and type sudo chmode +x (fpvfreerider.app) and that should do it.
It should look something like:
chmod +x "/Applications/FPVFreerider.app/Contents/MacOS/FPVFreerider"
or
~ chmod +x /Applications/FPVFreerider/Contents/MacOS/FPVFreerider
(adding sudo at the start if needed).
After that you might need to go to your Mac menu System Preferences>Security & Privacy
If it says something like "Freerider was blocked from use because it is not from an identified developer" you need to select "Open Anyway".
Hope that helps!