In checking with our team here at this time Visitors is not a metric that is available in this report and is not something that can be enabled. When -Visitors- is showing in Site Catalyst it is always referring to -Daily Unique Visitors- For example if you using this metric in a conversion report it is showing you daily unique visitors. This is not the case in Discover. Discover visitors is based on the time frame of the report you pull on whether it is showing daily, weekly. monthly, etc. This is the same with Data Warehouse. I would recommend making a feature request. The best way to make these types of feature requests is by having our clients go to ideas.omniture.com. These requests go to our product management team. Clients have the ability to vote up these ideas and they are strongly considered for future releases.
I have provided a knowledge base article that elaborates-
Why does Discover have "Visitors" whereas SiteCatalyst has "Daily Unique Visitors?" (Article ID 1528)
Discover handles Unique Visitors like Data Warehouse handles them. That is, it will take any time frame for which you run your report, and give you a true Unique Visitor count for that time frame. Therefore, in Discover, a time distinction is not needed on the metric name. Granularity takes precedence over the time frame for the line items in the report. What this means is that the line items, when summed, will not equal the sum at the bottom of the report. A good example of this is the Traffic Unique Visitors Report. If you have a month selected, the line items show Daily Unique Visitors, but the sum at the bottom shows Monthly Unique Visitors. As such, the report is showing two separate metrics.
I’ll go ahead to close this ticket. However, please feel free to reply to this email to reopen the ticket if you have any further questions or concerns regarding the items we discussed on this incident.