hi jaime,
look again in your widgets admin page. that’s where it should be. you can find your widgets in admin under the appearance menu item.
once you are there look on the right side of the page for a “left sidebar” section. open the left sidebar section using the dropdown arrow which should reveal an aweber widget. if you can’t see something called aweber, look just below the facebook widget and above article categories.
then open the aweber widget and click delete.