The chmod +x command adds execute permissions to the specified file, allowing it to be run as a program. It usually works to get Freerider running on a Mac. Make sure you enter the correct folder path and app name (where the app is located on your computer).
There is a detailed description on how to use it on page 16 of the user manual.
(If you still can't get it working please get back with more info on what happens when you try to run it).