Hi
Add-in is a software program that extends the capabilities of larger programs. For example, there are many Excel add-ins designed to complement the basic functionality offered by Excel.
What do you want the potx file do with? You want your PowerPoint start with this template?
If so, follow these steps:
1.Choose File | Save As.
2.Click Computer then click Browse.
3.Choose PowerPoint Template (*.potx) in the Save as type: dropdown list box. PowerPoint automatically takes you to C:\Users\[YourUserName]\Documents\Custom Office Templates. Which isn't what we want. So ...
4.Browse to C:\Users\[YourUserName]\AppData\Roaming\Microsoft\Templates instead. Substitute your user name for [YourUserName].
5.Name the file Blank (or Blank.potx) and click Save.
6.Try it out: Choose File | New and choose the new Default Theme to make sure PowerPoint uses your new template.
Regards
Tylor Wang
TechNet Community Support