If you do not see the option to select to enable NT Service Access within WebDrive (Site Properties --> Advanced), then WebDrive is not sensing the proper permissions are available in order to set this particular setting. You need to have Local Admin privileges in order to set this option. The simplest way to be able to set this setting is for WebDrive to be installed by a Local Admin account (not just Domain Admin).

If you are using Windows 10, you might have to start WebDrive using Run as Administrator and then set the NT Services setting in the Site Properties --> Advanced tab. Once you have done this, exit WebDrive completely (right-click on the system tray icon and choose to Exit), and then open WebDrive again normally. You don't want to run WebDrive in an elevated context throughout normal use. 

For information regarding the NT Service Access setting itself, please see the Help File in WebDrive.