This is only for new users of Airbnb. As far as I know there is no easy way to get free travel credit if your already a registered user of airbnb. If you have not linked your account to Facebook it is possible that you can sign up with a new email address and get another £30 free credit.
There are terms and conditions such as that you need to make a booking of £55 or more the use the free credit. The amount of the free credit has varied from time to time from as low as £15 to the now high £30. There is also a bonus for new hosts but this has been wound back recently. It was the case that if you referred a new host that you received £90 in cash from Airbnb but again this has been wound back and you now get £55 in travel credit.