Unique Visitor reporting in Urchin 4 requires the use of the Urchin Traffic Monitor (UTM) which is included in the base product. The UTM requires special configuration and settings which are detailed in the White Paper on "Urchin Traffic Monitor (UTM)". Please see the white papers section of the Urchin help center for details.
If the UTM is not installed, then Unique Visitors in the reporting will be zero. Once the UTM is installed, new log data will include unique visitor data. Historical data that did not include the UTM will always be zero.