I'm attempting to install this program to the Sims Legacy and the first issue it ran into is that it cannot find the files. I can direct the program to the files, but do not know what to put in for the Registry. Does anyone know what pathway to use?

The registry entry is pulled from the INSTALLPATH (I think that's the spelling) entry in the registry. You can't actually populate that field in the program (at least not without editing the registry). That field just allows you to reset the program's path to where the registry says your Sims game is. Sims Legacy does not create that entry in the registry when installing.

Are you asking for help editing the registry? Does the program not work if you simply point it to the files?

VerDeTerre, are you talking about the "Location of the Sims found in the registry" line? As far as I understand, it's important to have the right game path in the "Location of the Sims used by SimCategorizer" line. So just put your game path there.
I don't have the Legacy edition, but the info from the other people here is the following:
If you own a copy of The Sims Legacy on Steam: C:\Program Files (x86)\Steam\steamapps\common\The Sims Legacy Collection
If you own a copy of The Sims Legacy on the EA app: C:\Program Files\EA Games\The Sims Legacy

Yes, I cannot run the program without filling in the information asked about the registry. I'm not up for it right now, but I think I could try to put in the game file location in that field and see what happens.

There were actually two areas asking for location - the one you are giving was the second one which is asking the location of the files. The first one asks for the registry. I do not remember this from before and I think that's because it could find the pathway easily. With the Legacy edition, everything has changed.

Well, I don't think that the "Location of the Sims found in the registry" line asks you about anything. It tells you the information that the program was able to find without your help, reading it in the registry, and I don't see the option to edit it somehow. I've just tried to delete my game path from the registry and it still doesn't allow me to edit this line, but suggests to direct the program to the game manually instead. The second line doesn't really ask anything either, but gives you the opportunity to make the program use any other location then the one from the InstallPath key. There is the note there telling you that if it's impossible to find the location in the registry, you can use the second line to direct the program to the game manually. This option is specially created for some game versions without the InstallPath key, so I don't understand why it doesn't work without it. What really happens if you put your game path there? Do you run the program as an admin?