This is not as intuitive as it seems like it should be. I hope it gets better in RTM.
Like many companies, Aptillon has a series of Word and PowerPoint templates that we use as the basis for many of our documents. In previous versions of Office, simply placing the *.dotx or *.potx files into a specific folder (by default C:Users[UserName]AppDataRoamingMicrosoftTemplates would automagically make them show up as available templates (the UI varied slightly across Office products, but was pretty consistent overall.)
Apparently that changed in the Preview of Office 2013. Placing the files in that location has no effect. Instead, you need to configure each application to give it a Personal Template location. You can do that by opening the application (PowerPoint in this example), clicking File|Options|Save and specifying a path for the Default personal templates location setting:
Any template files you place in that folder (in the above example, D:Templates) will now be available for use from File|New|Custom|Templates:
Word works similarly. I didn’t try any of the other Office applications, but I would assume that they are similar.