you put that code wherever you want to allow the user to submit an application.
To me, it makes the most sense to make available on the actual job posting.
out of the box, the master plugin has a template file called job-application.php, which controls the information that shows up when you press "Apply for Job" button on the job posting.
However, you may need to take away portions of the code to make it work with your own theme depending on how your theme works. Using the standard installation of Genesis, mine had trouble locating the plugins class. Just an FYI.
You can put that code in that spot of the template file. You may also wish to comment out the standard code that tells the application to submit via email.
The only other issue I see with this add-on for the job manager is that it doesn't do anything to link the form to the application. The only link you'll see is gravity form's native embed page link which shows the page slug where the form was entered. That's not real helpful for a full blow job listing.
To make this really helpful, you'd need some custom code to pre-fill a gravity form field with the job name / id.