When you open the Express Addin, the dropdown choices for categories is blank.
How to fix it.
It is possible that the database file is corrupt, delete it so Express Addin will download a new one.
1. close PowerPoint.
2. Open File Explorer.
3. Browse to C:/Users/[your username]/AppData/Roaming/PresPro
Inside the Users folder, make sure you open the folder with the same name as the Windows Login you use to sign into the computer.
If you do NOT see a folder named "AppData", go to the View ribbon in File Explorer and in the Show/Hide section, check the box next to Hidden Items to see it.
4. Open the folder "DB" and delete all of the files in it. Do not delete the folder named "DB".
5. Close File Explorer and Open PowerPoint.
6. When you open a new blank presentation, the Express Addin will download a new database file to replace the one you deleted. Try applying a design again.