Built, not switched on yet. Dropbox support is built but not switched on in current releases. It needs a Dropbox app key that today's builds do not include; once it is on, Dropbox appears in Settings › Cloud Sync and in a workspace's Connectors tab.
Use Google Drive, or export the file from DocFila and upload it to Dropbox yourself.
There is no single Integrations screen. Cloud storage is in Settings › Cloud Sync and in a team workspace's Connectors tab. Slack, Gmail and Salesforce sign-in is in an organization's Settings. Accounting exports are in FinFlow › Reports. Outbound webhooks are an action in Tools › Workflows.
Keep going with the document workflow that matches this page. Open the tool you need, then finish the file in DocFila.