If you look on the left, you'll see "Work Type". If you click the drop down menu, "Internships" should come up as one of the choices. A lot of Executive branch agencies probably just use "Apply at USAJobs" as a generic message. Our offices use USAJobs for paid positions but not for unpaid internships. The interns are hired by posting ads through a college. I did a quick search of USAJobs in the Des Moines area and nothing came up within 20 miles but 11 internships came up within 200 miles.
You may have better luck going to a "Government and Nonprofits" career fair or looking for messages about visiting government recruiters on bulletin boards or in publications.