Hello @Chris1239,
If the email refers to the credit as a coupon then you have to click on the link on the payment page to use the coupon code when making a reservation.
If it's not a coupon and is travel credit from referral or from referring somoen to Airbnb then there are conditions on its use. This type of credit should be automatically applied by Airbnb if the reservation is a qualifying one. To qualify, the reservation must be for more than about US$75 before fees and taxes.
Coupons or referral credit can't be applied to an existing reservation or to long term, i.e. more than 28 nights, reservations.
If the above doesn't explain the failure to apply the credit then you should contact support.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.
