the small yellow one is the one to use. it does not go in all the way which worried me at first but, it found it to be snug and it stayed in reliably while i was in the field (all weekend at Vegoose.) Rat Shack MAY have a replacement that is shorter but, I haven't looked. (Let me know if you do!)
Be sure you have the polarity correct, i don't know what will happen if you don't but I know that it works when you do. Connect the jb3 to the side of the battery that does NOT connect to the wall. (thats the laft side if you are looking at the front of the battery.
As for the switch on the back, be sure it is set to 5v as indicated on the label on the back.
when hooked up, it should indicate AC power is in use. I ran without the internal batteries all weekend.
BTW this one does have a capacity indicator on the top (4 leds indicating, <25%, 25-50%, 50-75% & 75-100% respectivly). And check out the size.

(sorry for the sh*tty image qual)