Just run the app, and it will create the LaunchAgents folder and set everything up for you.
In case you wanted a more detailed answer:
When you first create a new user, ~/Library/LaunchAgents is not initially created. That's been the case since at least Snow Leopard (if I recall correctly). But that doesn't cause any problems. Once ~/Library/LaunchAgents is created, the operating system will happily start using it. The Keep Drive Spinning app will create the folder on the fly as needed, put the launch agent file into that folder, and then tell the operating system to load it for you. You never need to worry about moving around or loading the launch agent file yourself. That, after all, is the whole point of offering Keep Drive Spinning as an app. :)