What do you mean with "registration entry" ? A key in the Windows registry ? If the Addon Manager says it's Active, you surely have the key in the registry.
Or, you mean the .REG file that is created in your Home folder ? That is created only when you activate the scenery, so you surely must have seen it when you activate it, and it was created in your Home folder. If you don't have it now, the only possible explanation is that you might have removed it inadvertently, or it has been lost for some reason, like file system corruption, but it's 100% sure the file WAS created when you activate the scenery.
In any case, as explained many times on the forum, you don't really "need" that file. Is not used or read in any way by the software, the one and only use for it, is to save you from *manually* typing your Serial Number in a future reinstall, but that's it. It doesn't do anything else.
If you want to have it anyway, you can always use REGEDIT, go to the HKEY_SOFTWARE\Software\fsdreamteam\KIAH folder, and export the key into a .REG file.
