Hey man, I have the card that gives you the marvel starter pack for Infinity 2.0 as I purchased the Wii U version and the PC codes come on a card that I will not be using since I play it ion Wii U.
You put the code in here: https://infinity.disney.com/account#code-redemption and it will give you the Marvel starter pack with Iron man, Thor and Black widow and avengers playset, etc. Read that page and it explains it clearly how it works.
You get the PC version of the game here: https://infinity.disney.com/ for free and then just add these codes to get more characters and content.
It is not the steam version, but is the same game, steam does not have any achievements or steam cards for this game anyway, so it really does not matter. You lose nothing by it not being on steam.
If you are interested, let me know, thanks.
post here first before adding..