Make sure ubuntu has that nic set up and operational and probably you want it to use dhclient to obtain an IP.
If your host machine uses DHCP then your guest should as well. If you manually assign IP addresses for your host machine
you will need to do that in Ubuntu as well. You have to make sure that your router (or what ever) will assign the IP, that it doesn't have some hard coded limit or mac address filtering going on.
Check with ifconfig to make sure that the ubuntu nic has an IPV4 address.
Also in the VM settings, make sure that the Bridged Virtual Nic is set to connect at power on.