I tried to load that and I receive an Out of Memory message box...
It's made in a newer version of Construct. The latest version is available in the Construct Discussion forum, stickied at the top.
How is this?
I'm pretty sure he wants to be able to target multiple enemies, and that's just one enemy.
Here's a method for multiple enemies:
It's not perfect, for some reason every once in a while the missiles decide they don't want to pick a target. I don't really feel like tracking down why it doesn't work 100% right now. But it works most of the time, so it should be a good starting point to figuring out how to make your own missile tracking system.