Slide libraries, a new feature with PowerPoint 2013 are special types of document folders that store individual slides, not whole documents. To incorporate a slide from a SharePoint slide library into a presentation, just follow these steps:
Open the presentation you want to copy slides into

Do not open the presentation from which you want to steal the slides.
Click the Insert tab on the Ribbon and then click the New Slide button and choose Reuse Slides.

This step displays a Reuse Slides task pane.
Click the Open a Slide Library link in the Reuse Slides task pane.

This step summons a Browse dialog box.
Locate the Slide Library you want to steal slides from and then click Open.

The slides from the presentation you selected are displayed in the Reuse Slides task pane, in the same way that slides from a PowerPoint file are shown.
(Optional) To keep the original formatting for the slides, select the Keep Source Formatting check box.

Normally, you should keep this option deselected.
Click the slides you want to steal.

Each slide you click is added to the presentation.
When you’re done, click the X at the top right of the Reuse Slides task pane to dismiss it.