In AWeber, check that your list has at least one custom field in List Options -> Custom Fields.

If not, create a new custom field with any name. This is a placeholder that will be removed later.

In StealthSeminar, select and sync the fields you'd like to send data to.

In AWeber's Custom Fields screen, click Save.

The list will refresh, and the fields that Stealth Seminar created should appear. You can now remove any placeholder field you created.