If you have set up Aweber with your Optin Box, you may have received an error when testing that looks like the one below.
"Mailing List Not Active" is an error returned by Aweber, and refers to an invalid list name or list ID.
First, you should only be inserting information under Theme Settings → Optin Simple if you have an Aweber or GetResponse account. If you do not have an account with one of these autoresponder services, then you should not attempt to use the Optin Box.
Next, for your "Aweber List Name", add your Aweber Unique List ID. The Unique List ID can be found inside your list settings in Aweber.
Aweber has more information on your Unique List ID here.
More information on types of Aweber form errors can be found here.