The Visitors and Sessions report tab should only be viewable if you are using the Urchin Traffic Monitor (UTM) to track visitors. However, in versions prior to 4.1.04, it is visible even if the UTM is not in use and there will still be little or no data displayed.
The Visitors and Sessions report primarily displays data about Unique visitors. You must use the Urchin Traffic Monitor (UTM) if you want Urchin to track unique visitor traffic and populate this report with data. When the UTM is in use, Urchin can determine how many of the visitor sessions are new visiotrs to your website and how many are returning visitors. If you do not use the UTM, Urchin can only calculate the total visitor "Sessions." The total visitor sessions is displayed under the "Traffic" report. You can also view the total visitor "Sessions" under the Visitors and Sessions; "Sessions by day" report. All other report views under Visitors and Sessions require the UTM because they all display data about individual or "unique" visitors and not total visitor sessions.
Information about setting up the UTM is available here: Urchin Traffic Monitor (UTM)