In this Excel 2013 tutorial I will show you how to enable PowerPivot Add-in that is installed but is not enabled. PowerPivot is a com add-in and below are steps to enable it
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Excel 2010
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This tutorial should have very similar steps in Excel 2010
Video version:
NOTE: Ensure you have latest Windows Updates (you migh have to perform updates several times and include optional updates which may trigger new updates)
Excel 2013 Enable PowerPivot
First in my blank Excel workbook I click file

Then I click options

Next I get a dialog box where I click Add-Ins on the left from Manage drop down box I select Com add-ins and click Go button

Next I tick check box to for Microsoft Offie PowerPivot for Excel 2013 that will enable PowerPivot add-in and click OK button

You can see that new tab appeared called PowerPivot and on the screenshot below you can see the options I have available

That's all I hope this tutorial helped you to enable your PowerPivot add-in.
Take care
Emil