On the DirectAdmin homepage, you need to click on "Webmail" In the Extra Features Category;
You will then be brought to a screen where you can log in to your webmail account.
If you don't know them already, your username and password for this account can be found in the VMotion Client Area, which is accessed in the following 3 steps;
1: Click the "My Services" tab;
2: Select the product/service you wish to view;
3: Under the Login Details you should see your username and password;
4: You can now edit your Details;