In my experience the mailbox needs to be unhidden first. Once you have accessed it you can then re-hide the mailbox and still be able to open it thereafter.
In answer to your further questions below, yes this can be done. Once you have the resource set up and have set an owner for it, the owner can go into OWA in their personal account and from there open the resource account. There is an option to do this when they click the little arrow to the right of their name in OWA. Once the resource mailbox is open in OWA there are more detailed options as to who can schedule the resource. Have them click “options” while on the resource mailbox window and then scroll down on the left-hand side to “resource settings”. There they will find options to set who can schedule the resource as well as a place to set a response message. The response message there is for those who successfully book. All others who do not have permission to book a meeting will get a bounce back email telling them they do not have permissions. You do not have to set that one.