Did you already convert your json-string to a NSDictionary?
NSString *storeID = [(NSDictionary *)[json objectForKey:@"merchant"] objectForKey:@"StoreID"];
If not you need to convert it first:
NSString *json_string = @"{\"merchant\": {\"StoreID\":\"123\"} }";
NSError *error;
NSDictionary *JSON =
[NSJSONSerialization JSONObjectWithData: [json_string dataUsingEncoding:NSUTF8StringEncoding]
options: NSJSONReadingMutableContainers
error: &error];
NSString storeID = JSON[@"merchant"][@"StoreID"];
NSLog(@"storeID: %@", storeID);
// Log output: storeID: 123