If you have the Custom Animations Task Pane open, there is a Play button at the bottom which I think will do what you need as far as previewing goes. As far as Addins go the user would also have to have them installed on their machine.
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Michael Koerner
MS MVP - PowerPoint
Seeing as no-one has answered, let me be more specific.
I want to write an add-in for powerpoint that gives the user a host of
features that aren't supported by PowerPoint at the moment. I would like for
the user to be able to preview the visual look of a feature before running
the presentation.
Is there anyway to programmatically control what is displayed in the slide
pain (i.e. the pain where the current slide is being previewed/displayed when
not in slide show mode/presentation mode)?
The ways of interacting with PowerPoint that I am aware of is to use
vsto/vba to write an add-in or to interact with the object model.