What these people are trying to tell you is one of the cardinal rules of this place.
Dont pick a lock that is in use.
If its not in use then we would give you more help, but for all we know you could be trying to steal from someone. However, if the mailbox is just on your desk, like the handcuffs i have on my desk here, then by all means, send us a picture of the box and the lock and we can help you out.