To assign Email to a contact:
1. Select the folder from which to choose the email (usually Inbox)
2. Select the email (or emails) by clicking on the email
3. Drag and drop the email onto the contact
Note: If you press CTRL or Shift while dragging and dropping then a copy of the email will be created in the contact folder, otherwise the email will be moved to the contact folder.
After copying or moving the email to the contact folder, you can click on the contact (in 3 above) to show the emails allocated to that contact. In the address book you can also see which emails were allocated to a contact.