This could get really complicated really fast. PPT doesn't really "do"
dropdowns, but you could fake it. Again, real complicated real fast...just
be aware.

And there are a bunch of different ways you could accomplish what you're
after. Here's what I'd probably do:

Basically, have a trigger animation to trigger the entrance of the
"dropdowns." To follow your example, you'd add entrance animations to the
different references, and then start them with a trigger -- upon click of
"Reference Material." See http://www.echosvoice.com/triggers.htm for a quick
tutorial on creating triggers.

Then, once the different references are showing, each could have an action
setting applied that links to a different slide. To add an action setting,
right-click and choose Action Setting or choose Slide Show | Action
Settings.

So, you think thru it kind of backwards. Create your presentation so you
have all the slides. Then on the "menu" slide, you'd create different
autoshapes or textboxes to represent the different references. Add an action
setting to each reference shape/textbox to take you to the appropriate
slide. Then add an entrance animation to each reference shape/textbox. Then
make the animation start on a trigger -- upon click of Reference Material.

Just remember -- animations (including trigger animations) are "per-slide."
Action Settings and hyperlinks take you to a different slide.