You're not missing anything.
LivePacketDevice doesn't have a MacAddress property since winpcap doesn't support this.
Since Pcap.Net is a winpcap wrapper, it exports winpcap features and everything else are additions to winpcap's API.
However, since you've requested this, I've added support for getting the MacAddress from a LivePacketDevice.
I've added an extension method for LivePacketDevice called GetMacAddress().
It is in PcapDotNet.Core.Extensions dll and it uses .NET's NetworkInterface class.
I believe it should work (it works fine on my machine) and I hope it won't be too platform specific.
In order to use it you should do one of the following:
1. Wait for the next version of Pcap.Net.
2. Download the updated sources, compile and use it for yourself (recommended).
3. Use .NET's NetworkInterface class for yourself.
I hope this helps you.