To customize a panel:
Click the Settings button in the top right of the Nav Bar and select <Folder Name> Display Settings. For example, Mailbox Display Settings.
Select Panels from the View by drop-down list.
Enter the number of columns you want panels to display in.
Note that if you prefer using the QuickViewer more than two columns would likely not be useful.
Select from the list of available panels, or create your own panels by clicking New Panel.
Click Add next to the column you want the column to display in.
(Optional) Click Move Up or Move Down to position the column where you want it to display.
Click OK.