Look in Control Panel for Toshiba HWSetup. Click the LAN tab, and make sure Built-in NIC is enabled. If it’s disabled, Windows won’t see it in Device Manager. Click OK, and Windows will ask you to reboot. The NIC should appear in Device Manager after rebooting.
Is the NIC disabled in the BIOS? There could be some physical problem with the NIC. Try also having a cable plugged into the adapter when doing the installation. Do a search for new hardware also and see if any new devices are found.