As Brainwave says, but to be clearer. This is a project file. You cannot load it into another project file.
A template is designed to be used as a starting point and then you build your game around that template.
However, if you have an existing project that you want to integrate this into. Open both your existing project, and the template project, in the template project, select all the objects and copy them, then go back to your existing project and paste them in the objects folder.
Then repeat the same process for the event sheets, and layouts.
Now all of the content from the template will be within your existing project.
If you are using an outdated version of C3 and the template uses behaviours etc that only exist in newer versions this process will not work. However, I don't think this is the case with this template.