click on Get More , More , scrool down and u wil see Code Entry , paste code there and click Ok :)
Your answer is found here in detail:
https://gamehag.com/forum/t/20704-answered-how-do-i-use-marvelous-codes
in 3 steps
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I had a code from Marvelousga, i use it in steam but the steam said it invalid, can you tell me how to use the code?