How to achieve consciousness in nirvana? If achieving “nirvana” means that we will be liberated from the illusion of consciousness, how then will we know (be conscious of) our achieving this “nirvana”? How can we be “conscious” of this, if “consciousness” will no longer exist?
At the point of Nirvana consciousness ceases momentarily and starts again when you come out of the Phala Samapatti. At this point you see the the collapse and reforming of the links of Conditional Relations / Dependent Origination.

how then will we know (be conscious of) our achieving this “nirvana”?

You have seen the cessation of the 5 aggregates and start of it through full experiential knowledge of seeing the working of cause and effect.

How can we be “conscious” of this, if “consciousness” will no longer exist?

You need not be conscious about Nirvana but you insight into cause and effect will make a paradigm shift in your outlook about the 5 aggregates.
Also see: How does stream entry occur?
Also following quote might be of interest. This is slightly explained in more detail during the 20 day course.

What do you think is the ultimate goal of Vipassana?
SNG - The ultimate aim is to clean the mind. Nirvana is beyond mind and matter. It can come to you anytime depending on how pure you are. In that state, all your senses stop functioning. It could be for a second, a minute, or longer, but for that brief period, you are beyond all sensation, all thought.
-S N Goenka, from interview published in the 'life positive' magazine.