I just downloaded AutoDecline, but it isn't working. Just loads an empty box in game, and it hasn't been updated in some time. Is there another addon similar to this? To auto-decline duel and guild charter requests?

Try Damn Duel Decline! http://wow.curse.com/downloads/wow-addons/details/damn-duel-decline.aspx I don't know if it will work, as I don't use auto-decliners, but you can try it.

Or Darwinia's NoDuel http://www.wowinterface.com/downloads/info10762-DarwiniasNoDuelDDuel.html again, not sure if it will work

"leave me alone" is working as well - and has some nice extra features to go along with duel declining. The only thing i think it's missing is autodecline to guild charters.