To locate where PowerPoint expects templates to be stored, do the following:
1. Open PowerPoint and go to View > Master > Slide Master.
2. Go to File > Save As and click on "Design Template .POT" in the "Save as file type" box.
3. The "Save In" box will show the folder where PPT plans to save the file. Click the arrow on the right hand side of this box and the drop down menu will show you the full path to the folder where your templates are expected to be.
4. Exit PowerPoint and place new templates in the correct folder, or create a new folder at the same level and save them there. I like that method because I can have a folder called "\My Templates", for example.
5. Open PowerPoint and go to Format > Slide Design and you will see all of the available templates in the Task Pane.
The path to my templates:
You can include folders or create shortcuts to the templates in other versions of PowerPoint, or to other template collections you have purchased or downloaded.
Design Templates shipped with PowerPoint will be stored in the following locations. The paths on your system may vary, depending on where you have installed PowerPoint. The paths below are from my system, where I have 4 different versions of PowerPoint installed.