Requires login to download mods, but does not explain how to create a log in about modmyfactory2

[Artentus]

Downloading mods from the official server requires a valid factorio.com login. It's the same ingame, at some point in time you have to have logged in or you wound't be able to download mods.

Anyway, it's your factorio.com login that is required there, the same you'd use to log in on the website.

[BleuSquid]

I've never logged in to the factorio website before. I didn't know they had accounts, thank you. In-game identifies me by my Steam account.

[Artentus]

If you bought the game through Steam a "minimal" account was automatically created for you. You should be able to extend it to a full account somewhere in the games options according to the FAQ. Can't tell you anything more specific since I haven't done this personally.