Well, I know one thing for sure. Dark eldar drink wine of some sort. Not good for humans if I remember the story about the slave Gideon who had the "privilige" to meet Asdrubael Vect. There he was served wine that the human digestive system didn't handle very well, and it would be very painful... so, liquids of some sort are drunk.
When it comes to eating I've got no idea. I believe they might eat meat, plants, fish and the like (most probably things humans don't know or have).
I don't think the Dark eldar eat humans and that sort. If they did, I'd think the word for humans and such wouldn't have been "playthings", but "food".
I don't think they are cannibals either, allthough Dark eldar drinking blood wouldn't seem farfetched to me.