Trouble with Mailboxes in Mountain Lion

Try a vo back slash to expand and collapse the section you are trying to get to. Hope that helps.

I have the same problem. I've found the following steps can overcome it. 1. Navigate to the enclosing folder/mailbox that contains the mailboxes you're trying to access. 2. Interact with the title of the enclosing folder/mailbox using VO SHIFT DOWNARROW. 3. Navigate left and right until you hear an item called 'collapsed disclosure triangle', or something like that. The key word to look out for is 'collapsed'. 4. Activate that item by pressing VO SPACE. 5. Stop interacting with the title of the enclosing folder/mailbox by pressing VO SHIFT UP ARROW. 6. Navigate downwards and you should find the enclosing folder/mailbox has now expanded and is displaying the mailboxes you are trying to access. If this is not the case, take the following steps. 7. Navigate to the enclosing folder/mailbox that contains the mailboxes you're trying to access. 8. Move the mouse pointer to the item your VoiceOver curser is focused on by pressing VO COMMAND F5. 9. Check that the mouse pointer is focused on the enclosing folder/mailbox by pressing VO F5. 10. If the mouse pointer is correctly focused, press VO SHIFT SPACE to perform a mouse click, or click your Magic Mouse/Trackpad. If the mouse pointer is not correctly focused, repeat steps 7 through 9. 11. Now repeat steps 1 through 6. Basically, if it's not working, try to perform a mouse click on the mailbox you're trying to expand. Do this until a 'collapsed disclosure triangle' item appears. Then you can interact with the title of the mailbox and click/activate it. Best of luck!