When you start Designer Pro, you will be greeted with this dialog asking to activate:
Click on Connect to Xara Cloud to start. This will open your default browser to Xara Cloud, and ask you to sign in, if you haven't already signed in:
After signing in, if you have a valid trial or subscription, it will accept and Designer Pro + will activate:
And that's all there is to activating! Simply switch back to Designer Pro+ and it should be activated.