If you are running Mac 10.3.x, you will also have an ethernet tab to configure. The macs default is 'automatic'. Change that to 'manually'. You can then change your MTU from the default '1500' to 1392, or what ever value WBS comes up with next. If you are using the usb port, your configuration will show up under 'Ethernet Adaptor (en2)', hence the 'ethernet' tab to configure, even though you are using usb.