You can query CrateDB Cloud through the CrateDB Admin UI. In order to access it, go to your cluster "Overview" page, and click on "Open cluster administration":
In the next screen, you will be asked the username and password associated with your cluster (the 24-character password). If you don't remember those credentials, you can check out your username (and change your password) by clicking on "Settings".
Once you introduce your credentials, the CrateDB Admin UI will pop-up:
Once in the CrateDB Admin UI, you can write your queries on the console (second item on the left menu):