Magma Storm is a move that's exclusive to Heatran, and as such we can't use it yet in X/Y, however Serebii already has a page for it, despite it not showing up in the Attackdex's drop-down lists, much like with legendary-exclusive moves not called Psystrike, Geomancy, Oblivion Wing or Land's Wrath. Where it gets really interesting, however, is that he lists Magma Storm as now having 100 power instead of 120. This makes sense, considering pseudo-trapping moves got their chip damage doubled this generation, but... how does he know that? The same way he knew about Diancie, Volcanion and Hoopa and had location pages for them even before the hacker found them, most likely. This is something we'll have to keep an eye on once Pokémon Bank comes out. (Oddly enough, Psycho Boost is listed as still having 140 power, whereas Leaf Storm, Overheat and Draco Meteor are now down to 130.)
On a different yet related subject, how do pseudo-trapping moves fare now that their chip damage has been doubled? Well... Goodra actually uses Infestation on rare occasions, but that's it, really. So nothing changed on that front, and they're just as unused as ever.