Sorry Rags, what adapter? Did you get an O2 extender or some such? You don't need to do anything with the downstream sensor.
You can try reading this post
to find your wiring diagram and manual information for free from AutoZone. From there you should be able to find out where all of your oxygen sensors are. The wiring diagram will tell you if you have any other sensors besides the ones you've already found.
As to whether you will get a gain with HHO by itself. Usually people get some gain from HHO without doing anything else. However, it's an iffy proposition. Some people have even gotten worse mileage. The EFIE is needed, and to get the best mileage you should consider adjusting your other sensors as per Tuning For Mileage