Adding a harddrive/SSD can be a bit tricky if you haven't pulled things apart before.
Take a look at this video from OWC:
http://www.youtube.com/watch?feature=player_embedded&v=jSA65MjfRG8
It's the same for the 2012 model.
More info here for colonials:
http://eshop.macsales.com/item/OWC/DIYIMM11D2/
(People in the old world can go to http://www.macupgrade.eu for the kit)