MacBook keyboards hidden(combined) the function keys under Mac hardware keys like brightness control and volume control, so if you want use function keys we should hold special
fn key before pressing any of fn keys.
Being a developer I prefer to use fn keys without pressing
fn however when I'm not developing I prefer standard hardware keys like screen brightness and volume.
With this Alfred workflow you can toggle between both the keys right from wherever you are, give it a try.
Alfred Workflow - http://www.packal.org/workflow/toggle-fn-keys-standard-keys
Assign alfred keyword which starts ! or # for easy access to all your frequently used scripts at one shot, refer screenshot