Oh my! I don't know anything about using a doggie litterbox....but if you're trying to transition from that, just take the litterbox away (IMO). Start making it 100% UNacceptable for your dog to go potty indoors. Sully is 11 mo. old (born aug. 8, 2004) and he has been housebroken for about 2 months now (luckily, no accidents, knock on wood) - but whenever Sully pooped in the house, or peed, we'd stick his nose right up to it and yell "BAD DOG! BAd! Bad!" and the poor guy would cower in shame....but he remembered and woudl do it less.
Another idea woudl be going back to crate training for a month or two. Do a search for "house breaking" and crate training on this site...there are a lot of threads.
Good luck!