All of the templates download as ZIP files. If you are trying to open a template into photoshop directly from the downloaded zip file it will not work and it will give you an error.
You must first extract your files from the zip folder that you downloaded to be able to use them in Photoshop.
To do this simply right click on the zip file you downloaded and select "Extract All" and your computer will create a new folder and extract the files into it.
If you are still having issues please watch the video posed below on how to install your first template.