The code is: SDCCMH2015
I just tested this code and it is still working. See my screenshot below.
I'm not sure when this code will expire but if you are reading this post today (July 28, 2014) then it is still working. If you read this post a few days from today or a few weeks from today then still give it a try. Who knows it might still work.
Remember, I tested this code today (July 28, 2014) and it worked. Unfortunately, I can't guarantee that it will still work in the future because codes like this often has an expiration period.
For those of you who don't know, Marvel Heroes 2015 is a free-to-play superhero-themed action-MMO based on the Marvel Universe. The game will not let you create your own superhero but it will let you play with existing Marvel Heroes like Captain America, Thor, Iron Man, Spider-Man, Hulk, Wolvering, Punisher, Daredevil, Guardians of the Galaxy, X-Men and many more.
If you are interested in playing Marvel Heroes 2015 then check out the official Marvel Heroes 2015 website, learn more about it, create a free account, download the game and play.