Please take a look at the example you provided (it would say backendless.userService, not backendless.messagingService):
https://backendless.com/documentation/users/ios/ : “User Registration”
-(void)userRegistration
{
BackendlessUser *user = [BackendlessUser new];
user.password = @iAmWatchingU";
[user setProperty:@“email” object:@“james.bond@mi6.co.uk”];
Responder *responder = [Responder responder:self
selResponseHandler:@selector(responseHandler:)
selErrorHandler:@selector(errorHandler:)];
[backendless.messagingService registering:user responder:responder];
}