Really simple this one…
Go to File and then to Options.
Choose Addins from the list and then at the bottom of the resulting screen, you’ve got a drop down menu that says ‘Manage Excel Addins’. Select this! From the drop down listing select ‘Manage COM Addins’.
From the resulting menu, select ‘Microsoft Power Pivot for Excel’ and then ‘Ok’.
The Power Pivot tab will then be available in Excel.