To access additional fields when logging an email you will need to do the following:
1. If this is an inbound email, you would press the Log dropdown next to the email and then press Search and Select. For outbound emails, press the log dropdown and press Search and Select.
2. Select the Contact/Lead/Opportunity/etc. you want to attach the email to.
3. Press 'Review Selected Records & Options'
4. Fill out any of the fields that you need to and press 'Attach'