To show security tab at root of ESM:
Regedit HKEY_Current_User\Software\Microsoft\Exchange\ExAdmin
Add DWORD Key: ShowSecurityPage
Value: 1
Restart ESM
To allow access for administrators remove explicit deny.
E2K7 Powershell
Add-MailboxPermission Mailbox -user ServiceAccountMailboxName -accessright fullaccess -inheritanceType all