I just found a problem with GF and MailChimp groups. The good thing is that I also found a workaround.
When you define a group name in MC, it allows you to use "special" characters. When you create the feed, the groups appear fine. However, when you select one with a special character, the fields to select the condition don't show.
As soon as I changed the group name from "Básico" to "Basico" everything worked fine.