If you can get away with the simple textbox that pops up using the trick
that Lucy mentioned, that is the best solution. If you can get away with
a mouse click, instead of a mouse over, then you can use animation
triggers as another solution. However, if those don't work for you, you
are stuck with a macro. This is both very simple and very complicated at
the same time. It is simple because a line of code like:

ActivePresentation.Slides(3).Shapes(5).Visible = msoTrue

will make shape 5 on slide 3 visible. Put it in a macro and set the
mouseover function of another box run that macro, and you are set.

The complexities come first from identifying which shape you want to show
(you might be better off naming your shape than using numbers) and second
from making sure the shape is hidden again when you don't want it. This
will require figuring out when you want it hidden. For example, if you
want it to be hidden when you move the mouse off that shape, you will
need a shape surrounding your shape with the corresponding invisibility
code (just change msoTrue to msoFalse to make the shape invisible). If
you want it invisible when you come back to the slide, you can attach the
invisibility code to some other button.

Example 6.4 on my site (http://www.PowerfulPowerPoint.com/) is a simple
example of this. It doesn't use mouse over, but you could easily assign
the macro to the mouse over action settings instead of the mouse click
action settings.

--David