I don't think that Madara has a Curse Mark, but it is definitely a possibility. I couldn't see a Curse Mark back when Onoki took of Madara's shoulder and it revealed Hashirama's face. However, it is not a very good angle, since the Curse Mark is usually at the top of the shoulder or on the person's back.
As for how Madara could have a Curse Mark. The most likely situation is that Kabuto used what he had learned from Orochimaru to add a Curse Mark to Madara's body when he added Hashirama's cells. Kabuto had fused himself with Karin, Suigetsu, and Jugo's DNA, and it is entirely possible that he did the same with Madara's body. Since the Curse Mark originated with Jugo, Madara would then have a Curse Mark.