You would request the money directly in the resolution center. Never try to request money outside of the Airbnb website, it is a violation of your terms as a host and could get you and the guest in trouble.
Go to airbnb.com/resolutions
select the reservation you are dealing with
Select reason (You would choose "request compensation for damages"
Select "next"
you will then see this page...
Fill in the information and select "done adding items"
The guest will then have the option to accept the requested amount of money. If they don't airbnb will get involved and mediate the case. Do this right away as you only have 48 hours to put in the intitial claim against a security deposit.