At the top of your window go to Settings> Configure OpenLP>Presentations and make sure that PPT or/and PPT Viewer are checked. Just want to make sure you have that done first.
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Now, in the Media Manager on the left, select the Presentation tab, and then click the blue file button. If you hover over this it should say "Load a new presentation." Browse to your presentation and add it to the presentation window.
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Click the presentation you have loaded and then click the + button at the top of the Presentation window. If you hover over this button it should say "Add the selected presentation to the service."
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In the Service Manager window on the right, you should see your presentation. Click on your presentation to show it live.
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I'm not sure where you are in this process so I apologize if I have stated the obvious. But maybe this will help you confirm for us at what point you are having problems.