Where the arrow is...inside is a flap (normaly shut) will probably be covered in black give it a good clean (avoid droping anything in there, best use lint free cloth) carb cleaner or petrol is good to clean this, at the side where the throttle cable goes, if you move that it will open the flap so as you can clean round it alot easier...its very important that nothing drops in here!
Allow cleaners to evaporate and its possible that the car will not start first time.
If i have gone wrong could somebody please advise!