Cannot install downloaded/offline add-in from .VSTO file.

If the offline add-in has been downloaded from the Template Gallery via the About button

and it throws the following error when running the .VSTO installer:

Please carry out the following steps:

1. Make sure there is no add-in installed. Check in "Add/Remove Programs".

2. Make sure all instances of Word/PowerPoint are closed.

3. Open command prompt.

4. Run the following command:

    rundll32 dfshim CleanOnlineAppCache

5. After that has been run, try install the add-in from the files again.