If you do not see the option 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. 

One way to be able to use this setting is for WebDrive to be installed by a Local Admin account (not just Domain Admin) and then log in using this account when using WebDrive.

Otherwise, follow these steps: 

1. Exit WebDrive entirely (if running). Right-click on the system tray icon and choose to "Exit WebDrive". 

2. Right-click on the WebDrive icon or .exe file and choose to Run as Administrator.

3. In WebDrive, go to Site Properties --> Advanced set the NT Services setting in the Site Properties --> Advanced tab. Choose to have the system map the drive at startup.

4. Exit WebDrive completely (right-click on the system tray icon and choose to Exit), and then open WebDrive again normally, as you don't want to run WebDrive in an elevated context throughout normal use. 

5. Restart the computer.

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