I bought this game, but I cannot open it

See page 15 in the user manual: https://drive.google.com/file/d/0BwSDHIR7yDwSelpqMlhaSzZOa1k/view?usp=sharing

Basically, you need to open up the terminal and enter chmod +x (fpvfreerider.app)

It should look something like

chmod +x /Applications/FPVFreerider.app/Contents/MacOS/FPVFreerider

On page 16 in the manual there is a more detailed step-by-step instruction, if needed.