So what you need to do is go into the Mechlab menu (up at the top next to Home) go down to the bottom where it should show all 4 trial mechs. Click on the Centurion and just above it select ready. If the trials aren't there and all you have are empty mechbays then go over to the left and select Trial Mechs.
Edit: Should look just like this:
Except where it says configure mech it will say select mech. Just click on that and you should be good to go.