@Marc303 All the ways of contacting Airbnb are outlined in the CONTACT AIRBNB post in the forum you are actually posting in, I think about the third one down.
Have a look at the Airbnb host guarantee it will tell you how to make a claim.
If you are a 100% sure only the guest could have taken the case, then of course you can report the theft.
Airbnb expect you to contact the guest first before they will investigate. Just say you think they must have 'accidently' packed it and ask for it to be sent pack and give them a timeframe for it's return. Don't call your guest keep everything on Airbnb messaging so there is a record.