Paolo,
I think on some of these Models GE used a newer style of dispenser where a piston "fired" a dose of rinse-glo fluid through a hole that is underneath that large visible cap, but where the dishwasher spray can get at it and distribute it into the last rinse. As far as the black electrical tape, it could be holding the reservoir in place, or it could be a feeble attempt to plug a hole. You would have to very carefully remove it and see what condition the dispenser is in. If you're like the rest of Europe with ridiculously hard water, this dispenser would be useful, but if the city or town you live in happens to have soft water, it isn't necessary.
I'll look in some of my service manuals and see if there's a diagram of this type of dispenser. If I find it, I'll scan it and post it to this thread.