Steam Wallet

Download Arc and log in though it,add the card get the zen, go back to steam. It works had to do it myself.

From ingame when you go to purchase Zen during the checkout process steam will allow you to change the payment source do that and select your steam balance by default steam uses your primary payment source.

Sometimes it takes a while for Cryptic to get updated by Steam, usually if the store cannot see a recent wallet deposit it only takes a day or two for it to sync (and even that delay was rare when I did STO via Steam).

Of course, you need ARC anyway if you want to redeem codes so it doesn't hurt to have it. In my case, the game was having trouble with my motherboard software and I ended up switching completely to the non-Steam version to simplify the troubleshooting.

If the $50 is in your steam wallet you will need to launch the game through steam to proceed. From there you will find options in the game to purchase varying amounts of zen. Unless something has changed i'm not aware of, or having a massive bit of brain flatulence, you won't get any bonus zen. I must caution you that if this is your first time buying zen, buying it from a new platform, or is outside your normal purchasing patterns, you might trigger a temporary probation period on the account. This can last anywhere from a few minutes up to 14 days, though usually no more than 3 days that i've seen. This is an anti-fraud measure that's supposed to protect the company and you. If you use zen to purchase anything during that probation it will be bound to you for 7 days I believe it was, and then will become trade-able again. Hope that helps.