@rckoenes already showed you how to correctly get your data from the JSON string.
To the question you asked: EXC_BAD_ACCESS
almost always comes when you try to access an object after it has been [auto-]released. This is not specific to JSON [de-]serialization but, rather, just has to do with you getting an object and then accessing it after it's been released. The fact that it came via JSON doesn't matter.
There are many-many pages describing how to debug this -- you want to Google (or SO) obj-c zombie objects
and, in particular, NSZombieEnabled
, which will prove invaluable to you in helping determine the source of your zombie objects. ("Zombie" is what it's called when you release an object but keep a pointer to it and try to reference it later.)