NBA 2K12 on steam

Hi I'm new to the concept of modding games. I recently was able to purchase a steam key for nba2k12 and was wondering how could download mods into the game from there. I really want to try out the ultimate base roster and was hoping someone could give me a tutorial on how to load it into my file. Thank you!

how did you buy?

At this point, your best bet is a third party reseller or the grey market. Keys will still activate, but aren't being sold directly by Steam or 2K anymore.