Alexandra,
E-Prime 3 makes it much much easier to program this sort of thing. But
if you must do this in an earlier version of EP, then look at the
"Response Areas for Mouse Input" example that you may download from PST.
Bear in mind that PST examples often display poor programming
practices, so take the example as a starting point just to understand
the principles, apply your own wisdom to that, and build from there.
-- David McFarlane