Once you've turned on an email parser in your account, you have completed the only step that needs to take place in your AWeber control panel in order to have subscribers automatically added to that list.
However, since email parsers work by extracting information like name and email from an email sent by another, non-AWeber service, that service needs to be setup to send that email to AWeber.
Change Your Email Settings with Your Third Party Service
Let's say you're using a shopping cart or payment processor service that sends you an email whenever a purchase is processed. In your account with that service, there should be a setting allowing you to either change which (or add another) email address these messages are sent to.
Find that setting, and add your lists's email address.
If you have trouble finding these settings, search your service's documentation for information on your "notification email" or "email settings".
Once you've turned on your email parser and completed this step, your email parser setup is complete.