Complete Code

In case you need it, here's a copy of the whole ViewController.swift file at the end of the tutorial.

The Github project can be found at: https://github.com/ryancather/iOS-Swift-Calculator

 //
 //  ViewController.swift
 //  Swift Calculator
 //
 //  Created by Ryan Cather on 26/01/2015.
 //  Copyright (c) 2015 Lake Tuggeranong College. All rights reserved.
 //
 import UIKit
 class ViewController: UIViewController {

     var firstValue = true
     var subTotal: Int?

     enum operatorType {
         case plus
         case subtract
         case multiply
         case divide
     }

     var lastOperator: operatorType?

     @IBOutlet weak var calcDisplay: UITextField!
     @IBAction func pressEquals(sender: AnyObject) {
         var result: Int?
         if let finalOperator = lastOperator {
             switch(finalOperator) {
             case .plus : result = subTotal! + calcDisplay.text.toInt()!
             case .subtract : result = subTotal! - calcDisplay.text.toInt()!
             case .multiply : result = subTotal! * calcDisplay.text.toInt()!
             case .divide : result = subTotal! / calcDisplay.text.toInt()!
             }
         }
         calcDisplay.text = "\(result!)"
         subTotal = nil
         firstValue = true
     }

     @IBAction func pressMultiply(sender: AnyObject) {
         if let currentSubTotal = subTotal {
             subTotal = calcDisplay.text.toInt()! * currentSubTotal
         } else {
             subTotal = calcDisplay.text.toInt()!
         }
         lastOperator = operatorType.multiply
         firstValue = true
         calcDisplay.text = "\(subTotal!)"
     }

     @IBAction func pressDivide(sender: AnyObject) {
         if let currentSubTotal = subTotal {
             subTotal = calcDisplay.text.toInt()! / currentSubTotal
         } else {
             subTotal = calcDisplay.text.toInt()!
         }
         lastOperator = operatorType.divide
         firstValue = true
         calcDisplay.text = "\(subTotal!)"
     }

     @IBAction func pressMinus(sender: AnyObject) {
         if let currentSubTotal = subTotal {
             subTotal = calcDisplay.text.toInt()! - currentSubTotal
         } else {
             subTotal = calcDisplay.text.toInt()!
         }
         lastOperator = operatorType.subtract
         firstValue = true
         calcDisplay.text = "\(subTotal!)"
     }


     @IBAction func pressPlus(sender: AnyObject) {
         if let currentSubTotal = subTotal {
             subTotal = calcDisplay.text.toInt()! + currentSubTotal
         } else {
             subTotal = calcDisplay.text.toInt()!
         }
         lastOperator = operatorType.plus
         firstValue = true
         calcDisplay.text = "\(subTotal!)"
     }

     @IBAction func pressZero(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "0"
         } else {
             calcDisplay.text = calcDisplay.text + "0"
         }
     }
     @IBAction func pressNine(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "9"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "9"
         }
     }
     @IBAction func pressEight(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "8"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "8"
         }
     }
     @IBAction func pressSeven(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "7"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "7"
         }
     }
     @IBAction func pressSix(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "6"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "6"
         }
     }
     @IBAction func pressFive(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "5"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "5"
         }
     }
     @IBAction func pressFour(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "4"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "4"
         }
     }
     @IBAction func pressThree(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "3"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "3"
         }
     }
     @IBAction func pressTwo(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "2"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "2"
         }
     }
     @IBAction func pressOne(sender: AnyObject) {
         if firstValue {
             calcDisplay.text = "1"
             firstValue = false
         } else {
             calcDisplay.text = calcDisplay.text + "1"
         }
     }

     @IBAction func clearCalculations(sender: AnyObject) {
         calcDisplay.text = "0"
         firstValue = true
     }

     override func viewDidLoad() {
         super.viewDidLoad()
         calcDisplay.text = "0"
         // Do any additional setup after loading the view, typically from a nib.
     }
     override func didReceiveMemoryWarning() {
         super.didReceiveMemoryWarning()
         // Dispose of any resources that can be recreated.
     }
 }