With Mail open, click on the "Mail" menu, then click on "Preferences...".

Click "Add Account".

Select "POP" from the "Account Type:" box. Enter a description for the account in the "Description:" box. Enter the POP email address you created for your WebMail account in the "Email Address:" box. Enter the name you want displayed in the "Full Name:" box. Enter mail.yourdomain.com in the "Incoming Mail Server:" box. Enter the POP email address in the "User Name:" box. Enter the password for this POP account in the "Password:" box. Click "OK".

Close the Preferences window and you're done! You can now send and receive email using your new POP email account.