It's a subdirectory of the Application Data folder in your profile, and the
Application Data folder is a hidden folder. In order to see it in any dialog
within Word, you have to go into Windows Explorer and set it to show hidden
folders. That setting affects all of Windows, including Office.
Open Windows Explorer. Click Tools > Folder Options > View. In the Advanced
Settings list, click the option button for "Show hidden files and folders", and
click OK.
Now you should be able to navigate in the Save As dialog through the Documents
and Settings folder, then the one with your name, then Application
Data\Microsoft\Word\Startup.