In the yellow Events section, drag in a "when this sprite clicked" block. Make sure you've selected the cat sprite at the bottom right of the screen, or you may not see this block.
Change the size of the cat a little bit to make it bigger when it's clicked. Go to the purple Looks section and connect a "change size by 10" block to the "when this sprite clicked" block.
Go to the purple Looks section and connect a "change size by 10" block to the "play sound meow until done" block. If you want to undo the last change size, you can change the 10 to a -10.