Hi @Charlie6,
You cannot use the resolution center now since it has been more than 48 hours since checkout.
Do you have a security deposit option enabled? In that case you might be able to recover the amount, but I'm not sure if that too is time bound.
So you have pictures of the damage? I guess not since you've thrown away the mattress, otherwise, if you have evidence, you could contact Airbnb for some suggestions on how to recover the amount.
Imp. lesson learnt, detect problems asap and take pictures before discarding.
Good luck
Jeet