I think you're mixing some things up here. He never worked for the Stasi, on the contrary, he was under their observation since 1961.[1]
He was, however, a member of the Nazi Waffen-SS in his youth, though he claims to never have fired a shot. The fact that he chose to make this public only very late in his life (and many years after he had receiced the Nobel prize) is what is really problematic, in my opinion.
i never understood this part: how could he hide his Waffen-SS membership for so long? He would have had a highly visible tatoo on his left arm; well, maybe he got rid of the tatoo somehow after the war or something like that.
He was, however, a member of the Nazi Waffen-SS in his youth, though he claims to never have fired a shot. The fact that he chose to make this public only very late in his life (and many years after he had receiced the Nobel prize) is what is really problematic, in my opinion.
[1] http://de.wikipedia.org/wiki/G%C3%BCnter_Grass#Beobachtung_d... (the English article doesn't mention it, hence the German version)