How to move enemy towards a moving player?

You have to update enemy position and zRotation property in each update: method call.

Seeker and a Target

Okay, so lets add some nodes to the scene. We need a seeker and a target. Seeker would be a missile, and target would be a touch location. I said you should do this inside of a update: method, but I will use touchesMoved method to make a better example. Here is how you should setup the scene:

import SpriteKit

class GameScene: SKScene, SKPhysicsContactDelegate {

    let missile = SKSpriteNode(imageNamed: "seeking_missile")

    let missileSpeed:CGFloat = 3.0

    override func didMoveToView(view: SKView) {

        missile.position = CGPoint(x: frame.midX, y: frame.midY)

        addChild(missile)
    }
}

Aiming

To implement the aiming you have to calculate the how much you have to rotate a sprite based on its target. In this example I will use a missile and make it point towards the touch location. To accomplish this, you should use atan2 function, like this ( inside touchesMoved: method):

if let touch = touches.first {

    let location = touch.locationInNode(self)

    //Aim
    let dx = location.x - missile.position.x
    let dy = location.y - missile.position.y
    let angle = atan2(dy, dx)

    missile.zRotation = angle

}

Note that atan2 accepts parameters in y,x order, rather than x,y.

So right now, we have an angle in which missile should go. Now lets update its position based on that angle (add this inside touchesMoved: method right below the aiming part):

//Seek
 let vx = cos(angle) * missileSpeed
 let vy = sin(angle) * missileSpeed

 missile.position.x += vx
 missile.position.y += vy

And that would be it. Here is the result:

Note that in Sprite-Kit the angle of 0 radians specifies the positive x axis. And the positive angle is in the counterclockwise direction:

Read more here.

This means that you should orient your missile to the right rather than upwards . You can use the upwards oriented image as well, but you will have to do something like this:

missile.zRotation = angle - CGFloat(M_PI_2)