Administration of CATS has been moved to an NSF-funded site for concept inventories, which will ultimately be at:
If the final version of that site is not yet ready, from the http://cihub.org/ site, you can follow the link to their beta site: http://dev.cihub.org.
Several schools have used the beta site successfully to complete concept inventories. If you are an instructor thinking about having your students take a concept inventory, don't wait until the last minute: register right away as an instructor. (That includes selecting at least "Deploy concept inventories" under Site Interest.) Give yourself several days so they have time to verify your status as an instructor. Once verified, you will be able to view the questions of a given inventory, and then set up the time frame for your students to take that inventory.
If you have any difficulties or there are bumps along with way, the administrators of the site are more than willing to help out. If you have trouble, click on the help button and submit a support ticket.
Ultimately, cihub.org site will be a very valuable resource for instructors and students.