When the game first had the eyeballs it allowed me to put an aiming cursor on one using the mouse. After that.. nothing. I've managed to go on through the game for a while but I'm now stuck if I can't aim.

Can someone tell me how to get the aiming cursor to appear? Or is my game glitched? Or can you just aim at certain times?

you can auto target by hitting Tab.

Thanks. I had tried the tab key before but I guess I hadn't understood how the aiming works.

No problem, I did the same thing myself.