Hi Chema,
Mail Boss no longer manages any labels inside of your Gmail account as it no longer requires any access to your account. This was necessary due to new security restrictions put in place by Google. Also, unlike before, your scheduled messages are stored on Mail Boss servers rather than in your Gmail's drafts folder.
Regarding the extension, how it should work is that you create an email just as you normally would (with recipients, subject, and body) and then click the "Send repeating" which should send your message to your Mail Boss email address (where it will be stored until it's time for delivery) and close the "compose message" window. If it's not working, can you provide steps to reproduce the problem? Further, if you could send a screenshot or screencast of the problem that would be very helpful.
Thanks!
Dustin