Download the add-in file from the third-party source and store it in a location on your hard drive where you’ll be able to easily find it again. If you’re downloading the files from an unknown source, make sure that you have some type of antivirus software installed on your computer and that you scan the file before using.
Click on the Office button located in the upper left-hand corner of the PowerPoint window and then choose PowerPoint Options as shown in the screenshot below. (Click on the image for a larger view.)

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Once the PowerPoint Options window opens, select the Add-Ins category.

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At the bottom of this window, select PowerPoint Add-ins from the Manage drop-down box. Click the Go button to continue.

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When the Add-Ins box appears, click Add New.

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In the Add New PowerPoint Add-In window, navigate to the folder that contains the add-in you wish to include. Select the file and click OK to continue.

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It may take a few moments for the add-in to load and initialize. Once this is complete, click Close to exit the Add-Ins window.
If this is the first time you’ve installed an add-in, you’ll notice a new tab on the PowerPoint Ribbon for Add-Ins.

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Click on this tab and you should see your new add-in available for immediate use.
Additional Resources: For more tips, be sure to take a look at the other items in the collection of Microsoft PowerPoint user guides and tutorials found here on Bright Hub's Windows Channel. New and updated articles are added on a regular basis, so bookmark us and check back often.