I have declared an
-(void)parser:(NSXMLParser *) parser foundCharacters:(NSString *)string
element = string;
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.