The Calendar Designer consists of a Photoshop Actions file and a template file.
I would suggest that you copy the entire 2015 Calendar Designer folder and all it's contents into your Documents folder.
Templates:
The template file, 2015 Calendar Months.psd should be placed where you can easily access it. The exact location does not matter - just remember where it is. You will open this file every time you want to create a new calendar.
Actions File:
It really doesn't matter where this file is stored - just so we remember where. Again, I'd suggest that you copy the entire Calendar Designer Folder and all it's contents into your Documents folder.