To add your Bank X debit card on iPhone 6 and iPhone 6 Plus, open Passbook and tap the plus sign. Then select "Add Another Card" and follow the instructions.
To add your card on Apple Watch, open the Apple Watch app on your iPhone, tap "Passbook & Apple Pay", then tap "Add Credit or Debit Card" and follow the onscreen instructions.
Once your Bank X debit card has been added to Apple Pay, you will need to call our customer service to complete the provisioning procedure. Representatives are available Monday - Friday 6 am to 6 pm PST at 1-877-247-8018
For more instructions see https://www.apple.com/apple-pay/