You are right, there is no done button on the screen you shared. You just need to click on '<< App List'.
I am not sure why you do not see any devices within webCoRE though
Let me callout the steps I followed once again if it helps...
Go to the Apps page with the Hubitat portal.
If you have already installed webCoRE, you should be able to see it listed as shown in the image below...
Here, 'Hubitat webCoRE Director' is the name I had given to the webCoRE instance, at the time of installation.
Here, don't click on the parent level 'Hubitat webCoRE Director' link. Instead, click on the nested child level 'Hubitat webCoRE Director' link (Or whatever is the name you had given your instance).
On the following screen, you would see the Available Devices option shown in the screenshot you shared. Selecting it would take you to the Device selection list. Over here, do you see your devices already selected?