Scrolling down on the Admin Panel Mailng List > Compose Mailer page, you will find the KEY you can use to populate your mailing list fields into your mailer.  In the example on the right, the first two fields are connected to the Name and Email mailing list form fields, respectively.  The other fields are not set in the mailing list form and so are not set here.

Just follow the instructions printed under the KEY:
Type in the field KEY in your message and it will be replaced with each subscriber's value for that field before the email is sent to them. For example, to personalize each email message, if your first mailing list field is set to 'Name', in your email message type "Dear $field1,"