Check these things before you try the lemon. (One might be a trigger to allow you to do it.)
As you were coming up the stairs from the cave, did you turn on the rusty water valve?
In the temple, did you pick up the distilled water and transporter module Cl?
In the control room (the building opposite the temple), did you flip the switch on the center console? If I remember correctly, something lights up (the transport track?). The blue triangle is either on the console itself or on the wall. I can picture the room, but can't remember exactly where.
"Nanny Ogg looked under her bed in case there was a man there. Well, you never knew your luck."
-Terry Pratchett, Lords and Ladies