To add a Xero demo organisation to your Clarity account, click the blue + button in the bottom right hand side of your Clarity account. Select Xero from the options. Ensure you are either logged into the correct Xero account outside of Clarity, or logged out of all Xero account so you can log in and choose the correct one.
Click the blue Allow access button. If the Allow access button is greyed out, or not allowing you to select the demo company, open a new tab and log into your Xero account. Open or select the dummy organisation from within Xero so it opens. Come back into Clarity, refresh the page, and re-do the process. You'll be able to see the Allow access button is now blue and accessible.