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?
http://www.box.net/shared/yrvp2xyfjm
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:
http://dl.dropbox.com/u/529356/targeting.cap
uses v0.99.82
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.