The first time Web CEO 5 is launched, you'll see the "Unregistered" window with three available options: create account, enter registration info, or run in unregistered mode.

As you are already a registered user, you should click "Enter registration info" option to bring up the registration window:

Enter your e-mail and password (you can copy and paste them from your registration e-mail). Click "OK" to connect to the Datacenter.

Open the Project Manager. Please note that among your projects you will now only have those sites for which you have activated HitLens and/or Monitoring. Your next step will let you get the rest of your projects back.