I am unable to add the Custom chart app to a page as the "Add" button is grayed out.
The Custom Chart app is a Community app. In order to be able to install any community app, the Community App Catalog will need to be turned on for an entire subscription. This provides access to all community apps for all users in the subscription. To turn on the Community App Catalog within the subscription, the subscription administrator should go to the Setup menu, then select the Subscription tab in Actions, Edit Subscription, turn on the "Allow community app catalog apps" option. This will allow users to add community apps to their custom pages.