Virtualbox -> Can't add 2nd adapter to network for VM

Make sure the Virtual Machine is powered off. VirtualBox will not allow you to modify the network adapters while the VM is running.

If the VM is not running, and is in a saved state, you need to start it, then from the VirtualBox Manager, right click and select Close > Power off. You can't add a network adapter in the saved state.

Also, if required, remember to setup your proxy details in VirtualBox Manager. Power on the VM, then setup proxy details for IE.