I'm not super sure what you use it for but I think it connects to the steam battery. I think.
use the battery key
Well you go inside sullys but some robaot is going to start following you and it might help you well that's what i think lol <chispitaloverlol:P>
in front of sullys shop
you have to find the robot crab filled with water (near the door on top of sullys saloon) and use sprocket (in zacks garage next to sullys saloon) to drop it on the hatch and cool it down
click use by the sullys place. I havent figured out the combination let. Someone let me know of they have.
Sully's Paraphernalia is just left of main street
You need to get the mech steam motor and the mech crank. In a garage by Sullys is unfinished mech. Use your two mech items besides it and you then have built the mech.
you use the key
The mech is that thing that can walk around. It looks like it has 2 legs and you find in the garage next to Sullys. You'll need sprockets guidance in order to operate it.
there no key for poptropica
you use the key that the curator gives you on early poptropica and go back to the cottage and open the door with the key
You get the key from the bat, then there will be a back entrance, then you use the key there.