The most likely reason why you can’t select the network adapter in the Hyper-v Manager is due to the fact you most likely haven’t set up a virtual Network in the Virtual Network Manager.
Here’s the steps to setup a virtual network in Hyper-V.
Open the Hyper-V Manager and click on the the “Virtual Network Manager…” link in the far right Actions column.
Click “New Virtual Network – External – Add
Type a name into the Name field. If you only have one nic make sure that you select “Allow management OS to share this NIC”. Hit “OK”.
If you are prompted with any questions say yes. It’s going to disrupt the Virtual machine if it is running.
Go back to the properties of each virtual machine and make sure to select the network you just created and hit apply.
*NOTE* – Remember if you are installing a “dumb” OS where you can’t install MS Integration tools, you will most likely have to remove the current NIC and create a legacy NIC instead.