It's the room to the right and down a wee bit from the main buildings that had what appeared to be two water troughs in the middle. There's a sign at the "doorway" with an artist representation of what it would have looked like.
If you google Hadrian's Wall Latrine, tons of photos show up.
"We're facing a horde of ginger maniacs, with wild goats nesting in their huge orange beards-or to put it another way, the Scots!-and how does our inspired leader Hadrian intend to keep out this vast army of lunatics!? By building a three foot high wall!"
I've gone back and fixed the downloading option. At the time, I was uploading over a hundred photos, so I guess I forgot to make them all downloadable. Erg, now that's over a hundred photos to go back and fix.