macbook air: unable to assign static IP on USB ethernet adapter
Put a small switch between the device and the Mac.

Whatever "device" you're using, it's not bringing up the physical layer (OSI layer 1) and thus, your Mac is not seeing that anything is physically there.  That's why you're getting a "Status: Cable Unplugged" error message.

However, when you connect your Mac to a PC, you immediately get a self assigned IP address indicating that not only is a physical connection, but you made it all the way to Layer 3, the network layer.  You don't get an IP because there's no DHCP server, but you can now successfully set your desired IP and ping the PC.

A switch will between your device and your Mac will set bring up the link layer allowing you to set the IP.