If your copy of Word was issued to you by your university or place of business, you may not be able to install SmartCite (or any Add Ins) from Microsoft's Add In Store
If this is the case, you will not be able to install and use SmartCite in your copy of Word. However, there is an alternative. You may go to Office 365, create a free account with an academic e-mail address and use the free online version of Word with SmartCite.
To do this, head to Office 365 and create a free account
Once you've created that free account, you'll have immediate access to the free online version of Office and Word
Click on "Add Ins" and search for SmartCite in the Add-In Store
Once SmartCite has been added, click on "references" to access it, log in, and you're ready to go!
You can also use SmartCite for Google Docs!