Just pull up the carpet in the passenger footwell and you will see a silver metal panel,this is the compartment being referred to.It should be quite obvious at this point how to open it.
Cheers
This picture from the Gallery tells all. There is also a similar compartment under the drivers side carpet. It depends on the vehicle spec (additional radio amplifier etc) as to how much spare space is available on each side for storage. http://www.a2oc.net/gallery/showimage.php?i=458&c=10
When I visited a friend they said 'wow an A2 the drug smugglers car! '. I said 'what are you on about ?' and they proceeded to have a dig about in the footwells and hey presto YES a drug smugglers car!