It turns out that adding a Loopback adapter in Windows 7 is a little bit tricky, because we have no "Add Hardware" menu icon within Control Panel, anymore. Everything is supposed to be detected by Windows 7. (At least in current Windows 7 RC1).
But if we're talking about a "Loopback Netwrok Adapter", it really cannot be detected, as it is a virtual device..
In any case, if you want to run the Wizard where you can manually add hardware, you need to start it from the COMMAND PROMPT:
1.- Run cmd, but do it like: "Run as Administrator"
2.- From the command prompt, write down "hdwwiz.exe" and execute it. Then, the "Add Hardware Wizard" will be launched.
3.- Select: Install hardware manually --> Network Adapters --> Microsoft --> Microsoft Loopback Adapter.
You can read more info about it (step by step) in the following URL: