When you share a mailbox with another user, they can open the folders using File, Open, Others users... but what if you have subfolders in your mailbox? Only the default folders are listed in the Open other user's folders dialog.
In order to open other user's subfolders, you need to open the mailbox in your Exchange profile as a secondary mailbox. See How to view shared subfolders in an Exchange mailbox for instructions, including the permissions necessary on the mailbox root and subfolders.
If you are using OWA, you can view other users folders if you know the full path of the folder within the mailbox.
When logged into your mailbox with OWA, type the alias or STMP email address of the shared mailbox at the end of the URL. If you have the proper permissions, the mailbox will open. Should you only have rights to the calendar (or another folder), you'll need to add "/calendar" or the folder name to the alias. Remove the username and folder name from the URL to return to your mailbox.
You'll need to include the full path to the subfolder. (You may need to replace spaces in folder names with %20.)
For complete information, see Open Other Users Calendar with OWA