.. How can I unlock the Monkey Head?

You need the Monkey Head Key.

How can I get the key to the Monkey Head?

Now you can't just use the Monkey Head Key anywhere in the Monkey Head.

Where would you normally use a cotton swab?

In your ear.

The Monkey Head's ear is at the rightmost side of the Monkey Head screen.

Use the Monkey Head Key with the gigantic monkey ear.