1. Close Final Draft -- if you're prompted to save, don't save;
2. Open the Applications folder;
3. Right-click or Control + click on the Final Draft 11 icon;
4. Choose Show Package Contents:
5. Open the Resources folder;
6. Double-click on the first Courier Final Draft font icon...
6. ... and click Install Font:
7. Double-click and install the rest of the Courier Final Draft fonts.
After the fonts have been installed, open your script in Final Draft and it should be in the correct font.