NSMutableArray addObject not working
The "0x0" part is a memory address. Specifically, "nil", which means your mutable array doesn't exist at the time this is being called. Try creating it in your -init method:
categories = [[NSMutableArray alloc] init];Don't forget to release it in your -dealloc.
Initialize an empty array using
categories = [NSMutableArray array];The array class method are autoreleased so no need to release.