For example, if I click on STC Testing Center-Pecan Campus I see the following 3 choices.
After you click on the subject you wish to test on, you should see something similar to the below image. Now select the date and time you wish to take it on.
Make sure to look at the exact location of the test site. For example take the test sites below. These are two choices for the Pecan campus. One is in building M and the other is at the PECAN Plaza. Make sure to double check where the test site will be.