it's very easily accessible. the panel is not actually on the floor in the footwell but on the underside of the dash/glove box. stick your head in the footwell and look up. there's one black panel with two circular vents and then another further back (towards the engine compartment) - that's the one you want. there are two slots where you can insert a flathead screwdriver and release the catches. the panel can then be slid out towards the seat. you'll find the filter housed in a little compartment in the panel. my old filter was black, completely clogged and twice as thick as the new (white) one.

let me know if this isn't clear enough - i think i have a diagram somewhere i can dig out.