When you download it, it goes into your Templates folder. To make the
toolbar (which shows up in the Add-Ins ribbon tab) available later, you can
create a new document based on Sales Letter Toolbar.dot, attach that .dot
file to a document, or you can load Sales Letter Toolbar.dot as a global
template.
To create a new document based on it, choose Office - New - My Templates -
click on Sales Letter Toolbar.dot, and then click OK.
To attach it to an existing document, click the Developer ribbon tab
(Office - Word Options - Popular - Show Developer tab in the ribbon, if it's
not already showing), Document Template - Attach, etc.
To load it as a global, in the Templates and Add-ins dialog (Developer -
Document Template), click Add, etc.
In any case, it will then show up in your Add-Ins tab.