After logging in to Cpanel, click on the "Mail" icon. From there, you can choose to add new email accounts and modify old ones.
To check your email via webmail (Neomail, Horde, Squirrelmail), type in http://www.yourdomain.com/webmail into your browser. If a box pops up, click "Yes".
You will have to choose from 2 different email clients: Horde and Squirrelmail. Please note that for Horde and Squirrelmail username you need to use your FULL email address. For Neomail you only need the part before the '@' sign.