Here's a link to a photo I found a while back showing the engine serial number location and appearance.
http://www.rogerarrick.com/mustang/enginevin_800.jpg
For reference, it is on the top, driver's side rear of the engine, just behind the intake manifold. If your engine has been repainted, the numbers may be hard to see or feel. If this is the case, you may want to try doing a rubbing with paper/pencil to see if you can lift an image, or try using some detailing clay.
Just a caveat - I used detailing clay to verify mine, which did not quite give a clear impression of the number at first, but as it is pretty sticky, it lifted the paint, which then gave me a clear view of the number. The down side is the surface will then be exposed and may rust if not covered up again. The area is also somewhat hard to get at -- required some contortion for me to get my head back in there to see it.