Office 2007: Install an addin
Install your addin to your preferred location or as instructed by the author
Click
the Office Button
At the bottom of the Dialog click - Excel Options
to display this Dialog
Then in the next Dialog,
click Add-ins, 
At the bottom of the next Window you will see
Click Go and the usual Browse for add-in dialog is displayed. Select your add-in to make it available.
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