This is an asynchronous request, you have to set your class as delegate of the MGTwitterEngine and get the result in these method, that you must implement in your class:
- (void)statusesReceived:(NSArray *)statuses forRequest:(NSString *)connectionIdentifier;
- (void)directMessagesReceived:(NSArray *)messages forRequest:(NSString *)connectionIdentifier;
- (void)userInfoReceived:(NSArray *)userInfo forRequest:(NSString *)connectionIdentifier;
- (void)miscInfoReceived:(NSArray *)miscInfo forRequest:(NSString *)connectionIdentifier;
As you can read in the MGTwitterEngineDelegate.h:
// These delegate methods are called after all results are parsed from the connection. If // the deliveryOption is configured for MGTwitterEngineDeliveryAllResults (the default), a // collection of all results is also returned.
There you can check the result value.