Basically, you're going to be a lot better off just using the advanced direction extension for this.
If for some weird reason you absolutely *must* do it without extensions, the expression is this:
Clickteam really need to add ATan2 to all their current products, if they haven't already (not sure I have the latest build). Maybe if enough people email them or post on the clickteam forums...
Well incase it matters this works ATan2(x1-x2, y1-y2), or it least in the situation I need it to (set the angle of a nearby object to look at another).
help -> about
(or just look at the loading screen when you start MMF2)
It'll say something like "Version 2.0 (Build R249)"
Unless you downloaded it yourself, you won't have the HWA version.
However, it's well worth downloading, and it doesn't actually replace the standard version, so you can use both on the same computer.