OK, I think I get it. The aim of the project is to show that an optoisolator can turn a button on and off.
Which Optoisolator are you using? Both the input and output sides are polarity sensitive, unless the output is a triac, in which case it probably won't work with that low a voltage.
I presume this is an optoisolator included in the kit. Check your wiring again for proper polarity.