Discovery Search Mailboxes are arguably less used in Exchange 2013 (and Exchange online) than they were in Exchange 2010. This is mainly due to the additional options for handing search results available in the the Exchange Admin Center. An added factor to this is that the Mailboxes themselves can not be managed from the Exchange Admin Center, making it harder to both assign permission to the mailbox and make them visible so administrators can open them.
Should you wish to do these tasks, the syntax is:
To assign permissions:
Get-Mailbox –RecipientTypeDetails Discovery |Add-MailboxPermission –User email@example.com –AccessRights FullAccess –InheritanceType All
To make visible in address lists so can be opened:
Get-Mailbox –RecipientTypeDetails Discovery | Set-Mailbox –HiddenFromAddressListsEnabled $False