The XRK pack is already included in your purchase - no need for an extra code and contains the following two weapons for you to unlock:

XRK M4
XRK .357

You can unlock these two weapons from the Armory tab on the multiplayer screen. Interact with armor and you will see the XRK M4 under Assault Rifle and the XRK .357 under pistol.

To equip the weapons you need to reach level 4 so you have access to Loadouts. Once you’ve done this you can equip the XRK weapons into your Loadout by entering Loadout and going to Primary Weapon Select and the M4A1 (right over to Armory then equip XRK M4) and Secondary Weapon Select the .357 (right over to armory to equip XRK .357). All editions of the game received a Custom in-game Tactical Knife. This knife can be found in the Armory under the Melee tab. The knife is called Blue Silence.

To equip the weapons you need to reach level 4 so you have access to Loadouts. Once you are level 4 make your way to Loadout. Select the secondary weapon and highlight melee. Go to the right on combat knife to reach the armory and equip the Blue Silence.