Export mails in Unix Mailbox Format

I have an old email account archived in DT (folders with .eml inside) and I’d like to re-import to mail. To do this I need to be able to export in unix mailbox format but when I go to export as unix mailbox, I’m prompted to insert a file name but when I click save nothing appears in the appropriate folder, am I missing something?
Thx for help

eml-2-emlx: Free Mail File Converter
compatible with Mac OSX Lion (10.7)