I have a method that has a custom segue in my viewController that looks like this:
func gameOver() {
performSegueWithIdentifier(\"GameOver\", sender: nil
I don't have enough rep to reply to your last comment, but if you're still getting that error, that's because you haven't given that segue a name (or identifier) yet in Interface Builder!
When you click and drag to draw segues between view controllers in Interface Builder, you can select each transition by the circle icon that appears in the center of the segue lines and give it an 'identifier'. Then, when you call performSegueWithIdentifier, it should work!
This is what the circular icon might look like, depending on the type of segue:

If you're a little unsure of segues, check out this tutorial or this one!