It's on the left side of the screen.

Hint 1 of 6:

You'll first have to learn how to use the flagstone on the right side of
the screen before you can get the pin.

Hint 2 of 6:

Whenever one goblin jumps on it, the lid near the safety pin springs up.

Hint 3 of 6:

So if you put the other goblin on the lid, he'll shoot up.  But you need a
little more control over the lid to keep the goblins from flying everywhere.

Hint 4 of 6:

Maybe a little more weight on the lid would help.