Up in the upper right-hand corner where it has your user name next to "Inbox" and "Alerts", mouse over it. You'll see a drop-down list and in that list that has "Signature" as a link. Click on that "signature" link and add the banner code into that text box just like you would as a post.
As for the banner generator, if you can try it from another computer and it works, it may be a permissions setting restriction for the computer that you are using. (You mentioned in a prior post that you occasionally used public computers.)