From the Online Visitors list, click on the desired visitor's row.
The visitor's row expands to show the details.
From the Visitors Detail tab, click the Header column.
The values of the selected column are sorted.
The sorting works as expand sort. Which means that if you sort one column the values for other columns will also sort accordingly.
Lists are sorted in an ascending/descending or alphabetical order (depending on the type of values in the column you wish to sort). For example, incase you sort the IP column, the list is sorted in an ascending order; clicking the IP column again sorts the list in a descending order.