Here is the action that is called when you press an operation:

        @IBAction func operate(sender: UIButton) {
                let operand = sender.currentTitle!
                if (userIsInTheMiddleOfTypingANumber){
                    enter()
                }
                if let operation = sender.currentTitle{
                    if let result = brain.performOperation(operation){
                        displayValue = result
                    } else {
                        displayValue = 0
                    }
                }
            }    

Here is the code from the model:

func performOperation(symbol: String) -> Double?  {
    if let operation = knownOps[symbol]{
        opStack.append(operation)
    }
    return evaluate()
}

private var opStack = [Op]()
private var knownOps = [String:Op]()

init(){
    func learnOp(op: Op){
        knownOps[op.description] = op
    }

    learnOp(Op.BinaryOperator("✕", *))
    //knownOps["✕"] = Op.BinaryOperator("✕", *)
    knownOps["÷"] = Op.BinaryOperator("÷") {$1 / $0}
    knownOps["+"] = Op.BinaryOperator("+", +)
    knownOps["−"] = Op.BinaryOperator("−") {$1 - $0}
    knownOps["√"] = Op.UnaryOperator("√", sqrt)
    learnOp(Op.UnaryOperator("cos", cos))
    learnOp(Op.UnaryOperator("sin", sin))
    learnOp(Op.ConstantOperator("∏", M_PI))
}

private func evaluate(ops: [Op]) -> (result: Double?, remainingOps: [Op]){
    if !ops.isEmpty {

        var remainingOps = ops
        let op = remainingOps.removeLast() //get the first op off the stack

        switch op {
        case .Operand(let operand):
            return (operand, remainingOps)

        case .UnaryOperator(_, let operation):
            let operationEvaluation = evaluate(remainingOps)
            if let operand = operationEvaluation.result {
                return(operation(operand), operationEvaluation.remainingOps)
            }

        case .BinaryOperator(_ , let operation):
            let op1Eval = evaluate(remainingOps)
            if let op1 = op1Eval.result{
                let op2Eval = evaluate(op1Eval.remainingOps)
                if let op2 = op2Eval.result {
                    return (operation(op1, op2), op2Eval.remainingOps)
                }
            }

        case .ConstantOperator(_, let value):
            return(value, remainingOps)

        }
    }
    return(nil, ops)
}
func evaluate() -> Double? {
    let (result, remainder) = evaluate(opStack)
    println("\(opStack) = \(result) with \(remainder) left over.")
    return result
}

Note: I'm trying to complete the assignment's required tasks given the structure the instructor wants us to follow. My attempt here (trying to add ∏ as an operator) works in certain situations but not all...

Here is what the assignment says about this task: The value of π is available via the expression M_PI. E.g. let x = M_PI. You can think of π as an operand or you can think of it as an *operation** (i.e. a new kind of operation that takes no arguments off the stack but returns a value). Up to you. But, either way, it’d be nice to be able to add other constants to your Calculator with a minimum of code.*

I tried the operand approach but I was breaking the MVC paradigms so I stopped.