i was invited to come and start a pre-nysc job in one of the top ad agencies in the country.

i was told during the interview that i will not be paid until i start my nysc next year march as it be an avenue to train myself.
i was given one week to decline or accept the offer.

what should i do?

what is the probability that they will not trow you out after using you?they might want to use and dump you after you have served them.it does not even make sense working without pay

Will they give you feeding and transport allowance?
I think you should call the oga and ask if they will be kind enough to give you transport and lunch alowance

How far is the place from your location?
Considering it is a top ad agency (is it? seems everyone put top this, biggest that on every coy nowadays!)
it should be very good on your CV and if it in line with your career goal you will gain alot from it!

I did something similar, the company didnt pay me but the university did great for fresh grads if the conditions are right

Take it and learn the ropes