if you go in through peterson's, it won't work (I paid for 7 tests that way!)
Here is the air force link
US Air Force - Education and Library Resource Center
You will see info for CLEPs and DSSTs. Follow any links to CLEPs. Keep looking until you get to "take practice test now" type of option. I believe there is a list at the bottom of the page.
Once you enter, you'll need a code. The one I got here was FL4460. Follow the instructions to get to the test from there.
NOW, if you entered through the regular Peterson's and log in, the free test will be listed among any that you bought.
I have heard codes expire from time to time- so I went nuts. When I figured out how to add the test to my account, I went back and added 12 of them. They are good for a few months. You can always renew them if you need to.
One observation- if you renew them, they won't give you new unused tests- you'll have to keep your same "marked" tests. This may or may not be a problem for you.
Good Luck!!