Let x* be an element of (co)*. We show that there exists a sequence {ai} in l1 such that (see the pdf attachment).

I would let ei= (0,0,...,1,0,...) in co where the 1 is on the ith spot.

and x*(ei)=ai. we can show c0= closure of Span {ei: i=1,2,3,...}.

But after this I am completely lost.