How to restore a deleted mailbox located on your Mac?

To restore a deleted mailbox on your Mac.

Completely exit Mail.

Enter your Time Machine to a day/time that you know you still had your mailbox.

Cancel out of Time Machine.

Open Finder, select Go.

Hold down OPTION and select Library.  (You can release OPTION.)

Scroll Down to Mail.

Find the deleted mailbox.  It will have an extension of .mbox.

Drag it onto your desktop.

Open Mail.

Select File, then Import Mailboxes.

Select Apple Mail Format (yes, this is correct; do not choose .mbox format).

Follow prompts.

Mailbox will be restored, and will be at the bottom of all the mailboxes.  Drag it back into place.  

Ta da!