A family of Microsoft word processing software products for creating web, email, and print documents.
You don't add fonts to Word; you add them to Windows, and then they will be available in Word. Assuming you have downloaded the font, right-click on the font file and choose Install, which will copy the font to the Windows Fonts folder. Then when you restart Word, you should see the font. (In some instances, you may need to choose Install for All Users.)
Font substitution has nothing to do with this issue. When you open a document created with a font that is not installed on your computer, Word will substitute another font (it may bear no resemblance to the intended font), but this will not be obvious because the Font box on the Home tab will still display the name of the font originally used. This lets you know what font you need to install. To see what font has been substituted, go to File | Options | Advanced: Show document content and click on the Font Substitution... button.