My apologies in advance for a long response, but there are a few unknowns about your situation.
Question: is it your intention to have the text under your icons to appear only when the learner mouses over / hovers over the icons? Or are you looking to create a situation where, when the icon is clicked, additional information is revealed?
I ask, because the solution is going to be totally dependant on the scenario you want to create.
Here's the gist: In the case where "pop up" text is what you're looking for, it would be far better to develop that kind of functionality using Flash. The reason is that PowerPoint, through the use of Action Buttons or similar strategies, relies on code/programming that is not recognized or honored by Presenter...it simply strips that kind of functionality away when converting a PPT deck into Presenter content, but you could create a Flash element that could perform similar tasks...but again, it depends on what you're trying to accomplish.
One of the problems is that you'd have to create and code something like this in Flash....but there's another layer of complexity here: you stated that you're using icons for navigation. Navigating to what?
If, however, your intention is to have additional text elements appear IF something is CLICKED, then that's something you can do in PPT directly. Fundamentally speaking, the idea is to use PowerPoint's hyperlinking function to help navigate to a hidden slide....that hidden slide would be a duplicate that contains your icons (to help create the effect) along with the associated content you want to reveal.
Not sure if any of this helps, but I'm hoping something here will spark a solution to help you out.