You must remove the grille above the door. There are usually two screws just above the grille which you remove. Some grilles pull straight off. If it doesn't pull off, don't force it as some grilles slide to the left and then pull off. The charcoal filter will go in the center slot once the grille is removed. It just sits in there. Your unit may or may not have one in there already as the charcoal filter is considered an optional part.
PS: the grille on yours opens outward when you turn the fan on. It is still called a grille. These generally slide LEFT once you remove the screws above it.