After installing TB select to create a new account.
TB will ask you for your mail directory (When it's your first account), select your old mail directory.
Then TB will ask for your account name, give it the name of your old account and TB will recognize that it exists and take those settings and message base.

When you already created a new account, just copy the contents of your old mail directory on top of your new mail directory and start creating your old accounts back. Note that when you've named your new account the same as your old account you'd better rename the old account directory before copying it, because otherwise you would overwrite your new messages. When you renamed your old account dir, then you should use the new directory name as account name for creating it back.