There are five friction plugs holding the bottom of the grille in - they just pull out, but I would suggest that you be very carefull with the two at the outer ends, I suspect it would not be too difficult to break the grille.
Before you go adding refrigerant - what, other than the lack of cooling, makes you think it needs refrigerant? There are many possible reasons for the system to not cool, low refrigerant is just one, and if the refrigerant is low there's quite possibly a leak in the system that needs to be fixed.