Login screen - button to next segue

I've looked at a view other methods but not found one to do what I want and the ones that do carry out the task don't work for me.

I have 3 fields, all which require data to be entered to proceed to the next view, but it doesn't seem to work.

This is my code so far;

@IBOutlet weak var campustext: UITextField!
@IBOutlet weak var usernametext: UITextField!
@IBOutlet weak var passwordtext: UITextField!
@IBOutlet weak var loginlabel: UILabel!


@IBAction func loginbutton(sender: AnyObject){
    var campus = "Magee"
    var username = "B00123456"
    var password = "Password"

    if  campustext.text == campus &&
        usernametext.text == username &&
        passwordtext.text == password
    {
       let segueShouldOccur = true


    }

    else

    {
        loginlabel.text = "Oops..something went wrong. Please try again."
        println("Credentials were not correct")
        let segueShouldOccur = false


    }
}




override func shouldPerformSegueWithIdentifier(identifier: String!, sender: AnyObject!) -> Bool {
    if identifier == "mainview" {
        // perform your computation to determine whether segue should occur


        let segueShouldOccur = true || false // you determine this
        if !segueShouldOccur {
            let notPermitted = UIAlertView(title: "Alert", message: "Segue not permitted (better message here)", delegate: nil, cancelButtonTitle: "OK")

            // shows alert to user
            notPermitted.show()

            // prevent segue from occurring
            return false
        }
    }



    // by default perform the segue transitio
    return true
}


override func viewDidLoad() {
    super.viewDidLoad()
    // 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.
}

If anyone can help, would really appreciate it!