How to get my Green Party email address added to the CiviMail from dropdown list
This is a setting in CiviCRM setting under Administer --> System settings --> Outbound E-mail. 

There is a checkbox to allow different from e-mail addresses. If this is checkbox is set to no you can only send from a predefined list of senders. 

The from e-mail addresses could be found under Administer --> CiviMail --> From Email Addresses