To create your e-mail inbox in DirectAdmin please follow these instructions:
You have to chose User, in the top panel:
Then on the side panel, to create an e-mail address you have to chose E-mail Manager - E-mail accounts:
Or, if you want to send a vacation message, you have to go in E-mail manager - Vacation messages:
Finally, if you want to chose another name for your e-mail address (a forwarder, or alias), you have to go in E-mail manager - Forwarders: