In the Address Book, click New ▾ and select New Contact
Enter details for new contact and save
Highlight distribution list new contact is to be added to and then click Edit
In the Add contact… field start to type the name or email address of the contact created in step (2)
Select the contact from the drop down list shown
Be aware that the Address Book functionality is minimal, for example, exporting (backing up) and importing (restoring) breaks the link between contact and distribution lists; amending a contact's email address will no longer be automatically reflected in distribution lists they appear in.