Please first make sure, that the GAB has been synchronised to Outlook (Global Address Book (GAB) unavailable in Outlook). This is needed to search for and suggest usernames in the "Open folders for user" dropdown.
If no folders for your desired user are listed please check:
- If you have successfully selected the user from the dropdown. One indicator is a displayed full name.
- If you have the "show folders" permission on the root of the mailbox of the desired user. If you are not allowed to list folders, you cannot open them.