In case you still need it, here's a step-by-step tutorial my partner and I have on our website (with screenshots) that explains how to place your app screenshot inside a frame: http://blog.appdemostore.com/2014/10/how-to-add-your-screenshot-to-phone.html.
Basically, here's how to do it:
1. Open your Adobe Photoshop CS6. From the top menu, select File —> Open, choose your macbook psd and click Ok or hit Return.
2. Drag and drop your app screenshot to Photoshop, in the center of the macbook frame. Grab a corner, hold Shift and drag with your mouse in order to make the screenshot smaller. You can also move the image around to position it. Next, right click on the screenshot and choose 'Distort'.
3. Click & hold each corner and place it in its correct position. After you're done, hit Return.
4. Done. Save the final image from File —>'Save for Web...'.
I'm not sure if you are aware of the fact that you can easily achieve this task without Photoshop, by using online tools. This article lists some of the best online tools for this: http://www.versedtech.org/11293/add-device-frame-to-screenshot/ - such as such as picapp.net, placeit.net or appdemostore.com/frameapp. My favourite one is picapp.net because it's easy-to-use and free for small images.
Hope this helps. Good luck!