How do you use the custom parts?

You put the spectrobe that you want the custom part to be on, in the incubator, then click the icon that looks like a tool, and then it will point on the monster/s and you click him, then you will see the custom part and the monster, so click on the custom part and move it to the monster, then he will evolve with the custom part on him.

As for the key thing,1.It's called a keystone.2.U go through the 1st passage, through a path of sand,out up some stairs,up more stairs,into a green outer circle,into a another passage,u should go down stairs without that vrmp sound,down the room,into that 2nd chamber,then follow a path 2 another green outer circle(things that transports you 2 another area in that area),& you should see some different colored circley things colored blue at the top,green 2 the left,& red to the right.The keystone is at the top if you're getting it 4 Cyrus.If not,2 bad,no blue keystone.
Hope I helped,Rallen1997.