Before you can use your email you will need to create your email accounts from the DirectAdmin control panel. If you have not done this log into the control panel at http://www.YOURDOMAIN.co.nz/config and under the E-mail Management heading click "Email Accounts" then "Create Mail Account"
We have several webmail applications, have a play with each one and use the one you like the most:
Login using the email address you want to check and the password you created for that email.
My personal preference is for RoundCube, it is fast, clean and modern.
(Replace http://www.YOURDOMAIN.co.nz with the domain of your website)