If you’re using a storyboard, you probably shouldn’t be using alloc and init to create a new view controller. Instead, look at your storyboard and find the segue that you want to perform; it should have a unique identifier (and if not, you can set one in the right sidebar).
Once you’ve found the identifier for that segue, send your current view controller a -performSegueWithIdentifier:sender message:
[self performSegueWithIdentifier:@"mySegueIdentifier" sender:self];
This will cause the storyboard to instantiate an AddTaskViewController and present it in the way that you’ve defined for that segue.
If, on the other hand, you’re not using a storyboard at all, then you need to give your AddTaskViewController some kind of user interface. The most common way of doing so is to initialize the controller with a nib: instead of just calling init, you’ll call -initWithNibName:bundle: and provide the name of a .xib file that contains your add-task UI:
AddTaskViewController *add = [[AddTaskViewController alloc]
initWithNibName:@"AddTaskView" bundle:nil];
[self presentViewController:add animated:YES completion:nil];
(There are other (less common) ways of getting a view associated with your new view controller, but this will probably present you the least trouble to get working.)