On DirectAdmin, you should insert your DNS entry on the "TXT" line and then click on the button "Add".
If the previous service returns a success status and you saved your AcyMailing configuration with the DKIM parameters, then your e-mails will be automatically signed!
You can test it by sending an e-mail (from AcyMailing) to This email address is being protected from spambots. You need JavaScript enabled to view it..