VirtualBox networking can be hard to work with if you need your VM to access the Internet and be reachable from the host. If you can't use bridged networking (which we can't at NU), you need to set up two separate network interfaces.
In the main VirtualBox preferences, make sure you have a host-only network set up with the DHCP server enabled:
Create vboxnet0 (if necessary), then click the screwdriver to configure it.
When that's done, exit the VirtualBox preferences and open the Virtual Machine network settings.
Adapter 1 should be on the NAT network. This is the interface that will be able to reach the Internet through the host. You won't have to do anything special; the guest will use the correct interface automatically.
Adapter 2 should be connected to vboxnet0, where it will get an IP address in the range you specified in the last dialog box above. This will be the interface the host uses to browse the guest, and the IP address Avalon will be configured with.
After you complete the configuration and boot the system, run
ifconfig on the guest and look at the IP address of interface
eth1 to know what IP address to use from the host to contact the guest.