If you're on 64 bit Windows, did you consider the wow6432node translation node?
The registry entries should be under
HKEY_LOCAL_MACHINE\SOFTWARE\Westwood\Red Alert 2
but on 64 bit machines, the SOFTWARE key only contain the registry keys for 64 bit programs. The 32 bit ones can be found under HKEY_LOCAL_MACHINE\SOFTWARE\wow6432node\, meaning the RA2 key should end up at
HKEY_LOCAL_MACHINE\SOFTWARE\wow6432node\Westwood\Red Alert 2
If it doesn't exist... create it.
The actual value in there should be called "Serial", and if it doesn't exist you can just make it as new string value.
What's inside that value doesn't matter, as far as I know. I don't think the game itself actually contains any key validation; that was the CD installer's job. I'm fairly sure that as long as the value exists, and is filled in with a different value on all PCs, it should work for LAN.
