I know that this page is super old, but I have been having the same problem. I've been looking around the web for answers, and apparently, this supposedly missing dll file problem isn't a problem with the C&C games. It's a problem with Origin, in fact, I read an EA help page that said it happened with Mass Effect, and it wasn't even modded. I have no idea how to fix it, though. I tried to mod C&C: 3, and the same error occurred. When I modded Red Alert 3, it opened the base game with no changes. Modding Generals didn't work well either, so I installed a free third party version, which is totally fine 'cause I already own it. I wish Origin would stop being so annoying and restrictive. I'm sure that it's just so people won't cheat at games, but for games like C&C, EA needs to understand the purpose of modding. 💀

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Guys for the ultimate edition from Origin follow this video on youtube, it worked for me:
That user has more videos also for the normal version of Red Alert 3.