@Manon57
@Mike340 has summed up the alocation of coupons and credits very succinctly but, even though you did not get notification of a credit or coupon you may have received one. It pays to regularly look in your travel credit section to keep a check on what you may have.
You can find this section by Logging-on > Account Setting> Payment Methods and under Add Payment method you will see your travel credits section, as per this screenshot.....
Gift Cards and credits will come with a code, you simply put that code into the 'Apply to Account' box and need do nothing further. The amount will be automatically credited on your next trip.
With coupon codes you will need to insert the coupon code in the box next to the sub-total on the Check-out page and the value of the coupon will be deducted from the sub-total for that trip!
If you have been a Superhost for more than 1 calendar year and do not have a coupon showing in your travel credits you will need to contact Airbnb CS in France on +33 1 84 88 40 00 and they will be able to take care of this for you.
I hope this helps you Manon.
Cheers.....Rob