how do i redeem a code from a pack with pokemon cards

There are the following ways:

1) By Typing The Code in game

Launch the game and select head Shop tab.

Go to the "Redeem" tab.

Type the code of your Code Card directly in the "Enter Your Code" box. You don't need to insert dashes between letters.

Click the "Submit Code" Button, and as you do, you'll see everything you'll get in the "Rewards" Column.

Lastly, you must click the "Collect All" button. Then, the game will show you a screen showing everything you redeemed.

2) By Scanning The QR Code in game

If you have a webcam, you can scan the QR Code of the Code Card instead of typing it in the "Enter Your Code" box.

3) In official Pokémon TCG Live website

Also you can redeem in the official Pokémon TCG Live website. From the navigation menu, select Redeem Code, which will open the log-in screen for your Pokémon Trainer Club account. Log in to the account associated with Pokémon TCG Live. Enter your code(s) and select Submit Code. A menu should appear that displays your rewards. Select Claim Now to add the rewards to your collection.