I cannot add Content Manager after the new version came out and downloaded. I get the error message: The installation is only possible after unlocking Xara Web Designer Premium
Is your Update Service still current? If not, did you purchase the new version, either before or after downloading it?
I think the error message suggests the answer to those questions is No. Thus you have a currently unregistered installation of XDP 18 on your computer. Buying the upgrade will produce a new serial key which, when entered into the software, should ensure it is registered and you can download items from the Content Manager.