If the print result is null, it means that you didn't create the "mydata" NSMutableArray. Here is my code.

NSDictionary *dict = @{@"data":@[
                                   @{
                                       @"brand": @"AsafdsadfsdANTE",
                                       @"brand_id": @216,
                                       @"size": @"XL",
                                       @"size_id": @78,
                                       @"url_path": @"brands-1.html?brand=216&size=78"
                                       },@{
                                       @"brand": @"UYTIT",
                                       @"brand_id": @216,
                                       @"size": @"XL",
                                       @"size_id": @78,
                                       @"url_path": @"brands-1.html?brand=216&size=78"
                                       },@{
                                       @"brand": @"sFASDSADF",
                                       @"brand_id": @216,
                                       @"size": @"XL",
                                       @"size_id": @78,
                                       @"url_path": @"brands-1.html?brand=216&size=78"
                                       },@{
                                       @"brand": @"GERWW",
                                       @"brand_id": @216,
                                       @"size": @"XL",
                                       @"size_id": @78,
                                       @"url_path": @"brands-1.html?brand=216&size=78"
                                       },

                                   ],
                           @"status":@"success"

                           };
    NSMutableArray *myData = [NSMutableArray arrayWithObjects:@"A",@"B",@"C", nil];
    NSArray *data = dict[@"data"];
    for (NSDictionary *dict in data) {
        NSString *brand = dict[@"brand"];
        NSLog(@"%@",brand);
        [myData addObject:brand];
    }
    NSLog(@"%@",myData);

list of the brand values

the easiest way is:

NSArray *_brands = [responseArray[@"data"] valueForKeyPath:@"brand"];

that will gives you an array which has all values, but one brand can be multiple times in the final collection, so if the e.g. TRIUMPH brands are two times in your original data source, that will be two times in the _brands array as well.

every brand value only once

you can do this:

NSArray *_brands = [[NSSet setWithArray:[responseArray[@"data"] valueForKeyPath:@"brand"]] allObjects];

or even that:

NSArray *_brands = [responseArray[@"data"] valueForKeyPath:@"@distinctUnionOfObjects.brand"];

in both case, each brand will be only once in your _brands array like e.g. TRIUMPH, although it occurs two times it will be only once in the output collection, regardless how many times the data source has the same value; you can use whichever you'd prefer better or makes more sense to your project.

mutable collection

if you need really a mutable collection at the end:

NSMutableArray *_mutableBrands = [_brands mutableCopy];

Try like: if your array of dictionary contains key data, you can access directly:

NSMutableArray *addarra = [NSMutableArray new];

for ( NSDictionary *aDict in responseArray){
   NSMutableDictionary *temp = [NSMutableDictionary new];
  [temp setObject:aDict[@"brand"] forKey:@"brand"];
   [addarra addobject:temp];

}

Retrieve:

NSString *brand  = [[addarra objectAtIndex:i]ObjectForKey:@"brand"];