I don't use ARC because Sentinel does just fine for free, but if you want to monitor strictly only one site, create a favorites list, add the system from the main database into your favorites list, and then edit the favorites list, deleting all sites but the one you want. Keep the talkgroups, and in the system definition, you should see an option that lets you choose between ID SEARCH and ID SCAN. You want ID SEARCH.
When you save that after making that change, upload your data to your scanner, and it when it hits that favorites list during operation, it will scan strictly the site you didn't delete, and will show you all traffic. By not deleting the talkgroups you imported from the main database, you will have descriptions for the "known" talkgroups.