In order to redeem and open your MyTEAM card packs, you need to be in the MyTEAM mode of NBA 2K15, which is accessible from the main menu.
Once you're at the MyTEAM screen, bring up the menu and scroll over to "Rewards" and select it.
This will bring up the Reward Queue. From here, select any of the packs on the screen.
Once you've selected a pack, you can reveal its contained cards one at a time or all at once.