Nowhere. Marvel does not accept unsolicited submissions of any kind.

So your best bet, if you want to work at Marvel, is to go out there and create something, make your own comics, or novels, or teleplays, or whatever, and establish a meaningful track record. Then you can potentially interest a Marvel editor in inviting you to pitch on something.

But if you’re talking about just having a million dollar idea that somebody else will produce, and you’ll just sit back and count the money from being brilliant, sorry, don’t need it. We’ve got plenty of our own–and virtually all of such ideas aren’t as revolutionary nor as inventive as their originators think them to be.