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If you are using an outdated version of C3 and the template uses behaviours etc that only exist in newer versions this process will not work.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.
^^ What calminthenight said :).
If you want to take it to another level, look at every object closely; it's behaviors, variables, etc. Then when it comes to the event sheets, try to create your own event sheets in your own project.
Study the event sheets provided in the template project, follow it action by action as you add them. After all that you will learn how it all works together, to do what it does. Then, you will have better knowledge to create your own random levels in future projects :)
The event sheets part is easier if you have dual displays, you can open up the template project's event sheet on one display, then work on your own project on the other display. Slowly add each action on your own, exactly as it is in the template, and you'll begin to learn the process behind the template creators creation.
Good Luck with your project!