The Spark Web UI tab is available in the IDE is created when you start a Domino on Demand spark cluster. Directions for how to create a Domino On Demand spark cluster are located here for the 4.4 version:
The docs for other versions can be viewed by modifying the version from the docs page.
If you are using an external spark cluster then Domino does not have an internally visible dashboard available and you would need to contact the administrator of the spark cluster at your deployment to to get access to the jobs dashboard.
If you are creating a Domino On Demand spark cluster and the UI is still not visible to you, it's possible that the system has encountered issues while spinning up the spark cluster. The workspace logs should reveal some clues as to why the cluster is not spinning up, but if you are still stuck please reach out to your local admin or create a ticket with the Domino support team. If it is available to you, providing the Domino support bundle (https://docs.dominodatalab.com/en/latest/user_guide/055b74/support-bundles/) from your workspace will help the team find a resolution quicker.